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.
Perhaps the best way to deal with that is just to extend the language around "controversial" changes to include "**very** significant changes" so that the TC takes them up weekly. Then the TC can be somewhat subjective about whether or not the amount of review, and the people who were able to review it, were sufficient to merge.