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 think that if *everyone* (including existing committers and TC members) does non-trivial changes via a pull req, then the chances of a bad actor doing bad stuff is pretty low.

If someone violates that policy, then it gets reverted.  First time they get a warning, second time they lose access.  The TC can strip commit bit access at any time if someone's just generally being the poop.

The benefit of adding someone as a collaborator is that you have a high likelihood of that second (third, fourth, ...) patch coming, rather than having them just drift away.  And most people really want to be good and help out, once they're given permission to do so.

Low risk, high benefit.