Add a Contributing guide

CONTRIBUTING.md

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+# Contributing
+
+The documentation for each repository is available in the "**docs/**" directory of that repository.
+It is written in [Markdown format] and built using [MkDocs].
+
+## Testing Your Changes Locally
+
+We **strongly** encourage you to test your changes locally before contributing them.
+To do so, you first need to do three things; these are:
+
+1. Have MkDocs installed.
+2. Have the Laminas documentation theme available to the laminas-form directory.
+3. Update `mkdocs.yml` to build the docs using the documentation theme.
+
+To complete these steps, please follow the instructions below.
+
+1. [Install MkDocs].
+2. Install [the PyMdown Extensions].
+3. Clone the [documentation-theme repository] locally.
+    ```
+    git clone git@github.com:laminas/documentation-theme.git documentation-theme
+    ```
+4. Symlink the theme directory from the cloned documentation-theme repository, to the root directory of this project.
+5. Add the symlinked directory to .gitignore, to exclude it from being versioned with the project.
+6. Add the following Yaml snippet to `mkdocs.yml`, changing it to suit the repository that you are working with.
+    ```yaml
+    # The following example assumes that you are working with laminas-form
+    repo_url: 'https://github.com/<your-project>'
+    extra:
+        repo_name: <your-project>
+        # 'Mezzio' and 'MVC' can also be used as the project value
+        project: Components
+        base_url: https://docs.laminas.dev/
+        project_url: https://docs.laminas.dev/components/
+    theme:
+        name: null
+        custom_dir: 'theme/'
+        static_templates:
+          - pages/404.html
+    markdown_extensions:
+        - pymdownx.superfences:
+        - pymdownx.tabbed:
+        - toc:
+            toc_depth: 2
+    ```
+
+When you have completed each of these steps then, from the root directory of the project, run `mkdocs serve`.
+This builds the documentation locally, making it available on `http://127.0.0.1:8000`, if the port is not already in use, and actively watch for changes.
+If changes are detected, the documentation is automatically regenerated, saving you the trouble of having to do so manually.
+
+You should see initial output similar to the following if the process is successful.
+
+```console
+INFO    -  Building documentation...
+INFO    -  Cleaning site directory
+INFO    -  Documentation built in 2.47 seconds
+[I 200430 22:06:08 server:296] Serving on http://127.0.0.1:8000
+INFO    -  Serving on http://127.0.0.1:8000
+[I 200430 22:06:08 handlers:62] Start watching changes
+INFO    -  Start watching changes
+[I 200430 22:06:08 handlers:64] Start detecting changes
+INFO    -  Start detecting changes
+```
+
+When you view the documentation, it should be properly themed, similar to the following screenshot.
+
+![Example of locally generated Laminas documentation](/images/example-of-generated-documentation.png)
+
+## Need Help?
+
+If you have trouble with any of these steps, come and ask for help in [the Laminas Slack #documentation channel].
+
+[documentation-theme repository]: https://github.com/laminas/documentation-theme
+[Install MkDocs]: https://www.mkdocs.org/#installation
+[MkDocs]: https://www.mkdocs.org/
+[Markdown format]: https://www.markdownguide.org/
+[the Laminas Slack #documentation channel]: https://laminas.slack.com
+[the PyMdown Extensions]: https://facelessuser.github.io/pymdown-extensions/installation/