Add MR review policy

This adds an explicit policy describing expectations and responsibilities for MR review, based on a slight restructuring of the earlier draft. While this is phrased as a normative policy, obviously it is only my suggestion until it is merged, and I'm happy to make changes in response to feedback.

@simonpj @bgamari @mpickering I'd appreciate GHC HQ input into this. Perhaps we should circulate it to ghc-devs or more widely as well?