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 )
I would add this:

> Due to the need to load some of the proxy files asynchronously, you'll need to view the test files over the http protocol. On some systems, you can use something as simple as `python -m SimpleHTTPServer` from the root directory in this repository, then visit <http://localhost:8000/test/index.html> in the browser. Once there, be sure to click through each of the jQuery versions in the header to run the tests against each version. (If you have trouble running in different versions, make sure you are viewing `/test/index.html` not just `/test/`)