Add a basic CONTRIBUTING file.

Add a CONTRIBUTING file containing only the changes necessary for
contributing to this project, referring to the homebrew-cask project
for material common to both projects.

CONTRIBUTING.md

@@ -0,0 +1,18 @@
+# How to Contribute
+
+So you want to contribute to the project. **THIS IS GREAT NEWS!**  Seriously. We're
+all pretty happy about this. All of the [instructions for contributing from 
+the homebrew-cask project](https://github.com/phinze/homebrew-cask/blob/master/CONTRIBUTING.md) 
+apply, and won't be repeated here. The only changes concern getting set up to
+contribute:
+
+## Getting set up to contribute
+
+1. Fork the repository in GitHub with the 'Fork' button
+2. Add your GitHub fork as a remote for your homebrew-cask Tap
+
+```bash
+github_user='<my-github-username>'
+cd $(brew --prefix)/Library/Taps/caskroom-versions
+git remote add $github_user https://github.com/$github_user/homebrew-versions
+```