Add commiting guide

Author: kyleconroy

CONTRIBUTING.md

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+We love pull requests. Here's a quick guide:
+
+1. Fork the repo. Please also create a separate branch
+
+    $ git checkout -b my-cool-feature
+
+   We don't accept pull requests from master branches.
+
+2. Make sure the game runs. Follow the README to see how.
+
+5. Push to your fork and submit a pull request.
+
+At this point you're waiting on us. We like to at least comment on, if not
+accept, pull requests within a few days. We may suggest some changes or
+improvements or alternatives.
+
+Some things that will increase the chance that your pull request is accepted,
+taken straight from the Ruby on Rails guide:
+
+* Use Lua idioms and helpers
+* Update the documentation, the surrounding one, examples elsewhere, guides,
+  whatever is affected by your contribution
+
+Syntax:
+
+* Two spaces (or four), no tabs.
+* No trailing whitespace. Blank lines should not have any space.
+* Follow the conventions you see used in the source already.