Add contributing guidelines

- Based on https://github.com/thoughtbot/templates
- Closes #7

CONTRIBUTING.md

@@ -0,0 +1,44 @@
+# Contributing
+
+We love contributions from everyone.
+By participating in this project,
+you agree to abide by the thoughtbot [code of conduct].
+
+  [code of conduct]: https://thoughtbot.com/open-source-code-of-conduct
+
+We expect everyone to follow the code of conduct
+anywhere in thoughtbot's project codebases,
+issue trackers, chatrooms, and mailing lists.
+
+## Contributing Code
+
+1. Fork the repo.
+
+1. Install dependencies:
+
+    ```
+    bin/setup
+    ```
+
+1. Make sure the site builds successfully:
+
+    ```
+    bundle exec middleman build --verbose
+    ```
+
+1. Make your change, making sure the site builds successfully. Follow the
+   [style guide][style].
+
+1. Mention how your changes affect the project to other developers and users in
+   the `CHANGELOG.md` file.
+
+1. Write a [good commit message][commit]. Push to your fork. Submit a pull
+   request.
+
+Others will give constructive feedback.
+This is a time for discussion and improvements,
+and making the necessary changes will be required before we can
+merge the contribution.
+
+  [style]: https://github.com/thoughtbot/guides/tree/master/style
+  [commit]: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html

README.md

@@ -2,6 +2,14 @@
 
 [![Deploy to Netlify](https://www.netlify.com/img/deploy/button.svg)](https://app.netlify.com/start/deploy?repository=https://github.com/thoughtbot/middleman-template)
 
+## Contributing
+
+See the [contributing document].
+Thank you, [contributors]!
+
+  [contributing document]: CONTRIBUTING.md
+  [contributors]: https://github.com/thoughtbot/middleman-aria_current/graphs/contributors
+
 ## License
 
 Copyright © 2017 Tyson Gach and thoughtbot, inc. This is free software, and may