Add CONTRIBUTING.md.

.github/CONTRIBUTING.md

@@ -0,0 +1,6 @@
+## How to contribute
+
+* Please try to match the style of the code around the feature/bug you're working on. 
+* Use tabs not spaces. 
+* Please make sure your pull request only includes changes to the lines you're working on. For example, disable the whitespace extension when using Atom.
+* All pull requests must include code for every platform (Mac, Windows, and Linux) before they can be merged. The exception is platform specific features. Feel free to submit a pull request with code for one platform and others can fill in the gaps to help get it merged.