Add CONTRIBUTING

CONTRIBUTING.md

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+# 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 repository.
+2. Run `bin/setup`, which will install dependencies and create the dummy
+   application database.
+3. Run `bin/appraisal rake` to verify that the tests pass against all
+   supported versions of Rails.
+4. Make your change with new passing tests, following the [style guide].
+5. Write a [good commit message], push your fork, and submit a pull request.
+
+[style guide]: https://github.com/thoughtbot/guides/tree/master/style
+[good commit message]: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
+
+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.