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.
I believe, in the case where a single developer or group of developers has majority experience in a specific area of a contribution then effort should be made to make sure they sign off (e.g. if they were on vacation/etc. then the change should wait additional time).

Though this is for _very_ significant changes. e.g. I'd like to be notified if someone was going to restructure the buffer module.