docs: add contributors guide to aio

aio/content/guide/contributors-guide-overview.md

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+# Content Contributor's Guide
+
+Angular, as an open source project, depends on its community. This dependence is particularly important to the documentation. The more the community contributes to the documentation, the better the documentation becomes, which helps both new and experienced Angular developers.
+
+The topics in this section cover ways in which you can contribute to the Angular documentation set.
+
+## Before you begin
+
+Before you get started with your contributions, we recommend that you review [Contributing to Angular](https://github.com/angular/angular/blob/master/CONTRIBUTING.md#contributing-to-angular). That topic explains many of the tasks and guidelines you need to know before you make your first pull request.
+
+## Contributing to Angular
+
+<div class="card-container">
+  <a href="guide/reviewing-content" class="docs-card"
+    title="Reviewing content">
+      <section>Review content</section>
+      <p>Keep Angular content up-to-date by reviewing topics for accuracy.</p>
+      <p class="card-footer">Reviewing content</p>
+  </a>
+  <a href="" class="docs-card"
+    title="Updating search keywords">
+      <section>Update search keywords</section>
+      <p>Help Angular developers by improving the search keywords for existing topics.</p>
+      <p class="card-footer">Updating search keywords</p>
+  </a>
+    <a href="" class="docs-card"
+    title="Updating content through GitHub">
+      <section>Update content through GitHub</section>
+      <p>Learn how to make documentation changes through the GitHub UI.</p>
+      <p class="card-footer">Updating content through GitHub</p>
+  </a>
+    <a href="" class="docs-card"
+    title="Documentation Style Guide">
+      <section>Documentation style guide</section>
+      <p>Review the syntax and styles used within the Angular documentation set.</p>
+      <p class="card-footer">Documentation style guide</p>
+  </a>
+</div>
+

aio/content/guide/reviewing-content.md

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+# Reviewing content
+
+Angular developers work best when they have access to accurate and complete documentation. Keeping existing content up-to-date is an essential part of ensuring that all developers have a great documentation experience.
+
+This topic describes how you can help keep Angular content up-to-date by reviewing content.
+
+## Before you begin
+
+You can review content even if you've never contributed to Angular before. However, you may find it helpful to have the [Contributing to Angular](https://github.com/angular/angular/blob/master/CONTRIBUTING.md#contributing-to-angular) guide available if you're filing your first pull request the repository.
+
+## Reviewing content (`@reviewed`)
+
+All of the task-based guides, tutorials, and conceptual topics that you find on Angular.io support a `@reviewed` tag. When present, this tag is followed by the date representing when a given topic was reviewed for accuracy and completeness. On the published topic, this reviewed information appears at the bottom of the topic.
+
+<div class="lightbox">
+  <img src="generated/images/guide/contributors-guide/last-reviewed.png" alt="Example of the last reviewed date on a page footer.">
+</div>
+
+This reviewed date indicates when someone last reviewed the topic to ensure that its contents were accurate.
+
+You can review a topic using either the GitHub user interface or the command line on your local machine. You can also review any topic that you like. Previous experience in the subject of the topic is helpful, but not required.
+
+**To review a topic:**
+
+1. Navigate to the topic that you want to review.
+
+1. Locate the last reviewed date at the bottom of the topic and verify that the topic meets the [review criteria](#review-criteria).
+
+   If the topic does not have a last reviewed date, you are welcome to add it to the topic.
+
+1. Read through the topic.
+
+1. If the topic requires an update, either [file an issue](https://github.com/angular/angular/blob/master/CONTRIBUTING.md#-submitting-an-issue) that describes the update required, or [create a pull request](https://github.com/angular/angular/blob/master/CONTRIBUTING.md#-submitting-an-issue) with the update.
+
+1. Update the `@reviewed` tag, either through the [GitHub user interface](guide/updating-content-github-ui) or through Angular's [standard pull request process](https://github.com/angular/angular/blob/master/CONTRIBUTING.md#-submitting-an-issue).
+
+{@a review-criteria}
+### Review criteria
+
+In general, topics should be reviewed either every six months, or around every major release.
+
+On occasion, topics might require a more frequent review. However, we request that you do not file unnecessary pull requests for reviewing content.

aio/content/guide/updating-content-github-ui.md

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+# Updating topics through the GitHub user interface
+
+This topic describes how to submit pull requests to the Angular repository using GitHub's user interface. If you are unfamiliar with Git, you might find this process easier for making changes.
+
+<div class="alert is-warning">
+
+   Using the GitHub user interface for updates is recommended only for small changes, such as [updating the review date](guide/reviewing-content) or [updating search keywords](guide/updating-search-keywords).
+
+</div>
+
+**To update a topic through the GitHub user interface:**
+
+1. Navigate to the topic for which you want to file a pull request.
+
+1. Click the edit icon at the top of the topic.
+
+   <div class="lightbox">
+    <img src="generated/images/guide/contributors-guide/edit-icon.png" alt="The edit icon for an Angular topic.">
+   </div>
+
+   A GitHub page appears, displaying the source of the topic.
+
+1. Update the topic.
+
+1. At the bottom of the screen, update the Commit Changes box with a description of the change. Use the format `docs: <short-description-of-change>`, where `<short-description-of-change>` briefly describs your change. Keep the description under 100 characters.
+
+1. Verify that the **create new branch** option is selected, then click **Commit Changes**.
+
+  A Pull Request screen opens.
+
+1. Fill out the form in the Pull Request screen. At a minimum, put an `x` in the **Docs have been added / updated** option and the **Documentation content changes** option.
+
+1. Click **Create Pull Request**.
+
+At this point, your pull request is added to a list of current requests, which the documentation team reviews weekly.

aio/content/guide/updating-search-keywords.md

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+# Updating search keywords
+
+In documentation, being able to find the content you need is equally as important as the content itself. In Angular.io, users can discover content in several ways, including:
+
+* Organic search results, such as through google.com
+* The table of contents (also known as the left navigation)
+* Using the search box on Angular.io
+
+You can help improve the documentation experience by adding search keywords to a given topic. Updating search keywords can help bring users to the content they need faster.
+
+## Before you begin
+
+You can update search keywords for a topic even if you've never contributed to Angular before. However, you may find it helpful to have the [Contributing to Angular](https://github.com/angular/angular/blob/master/CONTRIBUTING.md#contributing-to-angular) guide available if you're filing your first pull request the repository.
+
+**To update search keywords:**
+
+1. Navigate to the topic to which you want to update search keywords.
+
+1. Decide what search keywords you'd like to add to the topic. For information on how to format keywords, see [Search keywords format](#format) for i
+
+1. Update the `{@searchKeywords}` tag, either through the [GitHub user interface](guide/updating-content-github-ui) or through Angular's [standard pull request process](https://github.com/angular/angular/blob/master/CONTRIBUTING.md#-submitting-an-issue).
+
+  If a topic does not have a `{@searchKeywords}` tag, you can add it to end of the topic.
+
+{@a format}
+## Search keywords format
+
+You add search keywords to a topic using the `{@searchKeywords}` tag. This tag takes a set of single words, separated by spaces. For example:
+
+<code-example lanugage="javascript">
+
+{@searchKeywords route router routing navigation}
+
+</code-example>

aio/content/navigation.json

@@ -959,6 +959,23 @@
               "url": "guide/styleguide",
               "title": "Coding Style Guide",
               "tooltip": "Guidelines for writing Angular code."
+            }
+          ]
+        },
+        {
+          "url": "guide/contributors-guide-overview",
+          "title": "Content Contributor's Guide",
+          "tooltip": "Describes how to contribute to the Angular documentation.",
+          "children": [
+            {
+              "url": "guide/reviewing-content",
+              "title": "Reviewing content",
+              "tooltip": "Keep Angular content up-to-date by reviewing topics for accuracy."
+            },
+            {
+              "url": "guide/updating-search-keywords",
+              "title": "Updating search keywords",
+              "tooltip": "Help users find the content they need by adding additional search keywords."
             },
             {
               "url": "guide/docs-style-guide",