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?

README.mkd

@@ -44,7 +44,7 @@ But being a member of the GHC Team carries extra power, as well as extra respons
 **Responsibilities**
 * Actively contribute to GHC, through patches, code review, design discussions, infrastructure.  "Actively contribute" means at least with a tempo of every month or two.  e.g. making a small intervention once a year is not really enough to be a member of the team.
 * Monitor, and when appropriate contribute to, the [`ghc-devs` mailing list](https://mail.haskell.org/mailman/listinfo/ghc-devs).
-* Willingness to proactively review merge requests, when the expertise fits.
+* Willingness to proactively review merge requests, when the expertise fits. See the [merge request review policy](./review-policy.mkd) for discussion.
 * Willingness to undertake some specific obligations, to take responsibility for some specific part of GHC