Add code of conduct to CONTRIBUTING document

[fixes #1967]

CONTRIBUTING.md

 Contributing
 Contributing
 ============
 ============
 
 
-We love pull requests. Here's a quick guide:
+We love pull requests 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
+
+Here's a quick guide for contributing:
 
 
 1. Fork the repo.
 1. Fork the repo.