Create CONTRIBUTING.md

Adding notes to get people started contributing to Version Cake, it's so easy!

CONTRIBUTING.md

@@ -0,0 +1,18 @@
+# Contributing
+
+We really like when people not only discover something they want added/fixed/removed in Version Cake, but also when they do it themselves. Here are some steps to get you on your way to contributing to Version Cake:
+
+* Fork the repo.
+* Setup for developing: `bundle && rake` - make sure all tests are passing
+* Add a test and make your change
+* Make sure all tests are passing
+* Add any additional notes to README.md or documentation
+* Commit/push/submit pull request
+* At this point someone will review, comment on your pull request and (hopefully) accept it very quickly
+
+# Test Other Supported Rails versions
+
+Version Cake supports multiple versions of Rails with Appriasals. These will be automatically tested against several ruby versions on CI. If you need to debug a problem, or want to be dilegent and run all of these do the following:
+
+* Install the gems for all supported Rails versions: `bundle exec rake appraisal:install`
+* Run all the tests for the supported Rails versions: `bundle exec rake appraisal test`