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.
An easy way to define a "controversial change" is if another TC member -1's a PR. But do we have an idea of when, say, the TC are all in agreement of a change but the community seems to think it's a dumb move and when/if that would ever be considered a "controversial change"?