Add contributing guidelines

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

CONTRIBUTING.md

@@ -0,0 +1,43 @@
+# 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:
+
+    ```
+    bundle install
+    ```
+
+1. Make sure the tests pass:
+
+    ```
+    bundle exec rake test
+    ```
+
+1. Make your change, with new passing tests. 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

@@ -42,6 +42,14 @@ detailing its usage][article].
 <a href="/about">About</a>
 ```
 
+## Contributing
+
+See the [contributing document].
+Thank you, [contributors]!
+
+  [contributing document]: CONTRIBUTING.md
+  [contributors]: https://github.com/thoughtbot/middleman-aria_current/graphs/contributors
+
 ## License
 
 middleman-aria_current is copyright © 2017 Tyson Gach and thoughtbot, inc.