First draft of a new contribution policy.

This is definitely not final, we'll need to discuss in the next TC meeting, but please comment prior to that.
we talked about this on TC call and people seemed to be in agreement that there wasn't much reason to try and restrict the commit bit provided everything is getting reviewed. would like to hear your thoughts though. here's the relevant part of the TC meeting although you may need to skip through Bert's technical difficulties with hangouts http://youtu.be/X5PtRHjUO1U?t=2m1s