Add 'contributing' documentation

Hey folks! This is the last bit I wanted to get in for version 1.6: documentation on how to contribute. Let me know if you have any comments. I'm sure I could add more info, but I didn't want it to look too daunting.

Any thoughts are much appreciated! ( @jcreamer898 @dcneiner )
Guessing you want `[QUnit](http://qunitjs.com/)` here.