Update MR Policy With Code Review

The CONTRIBUTE.md should be updated with a policy for Merge Request reviews.

Some MR's need discussion (ie: the single channel), some don't (ie: fixing typos) and some lie in between (ie: !59 (merged) ). It would good to have some guidelines to identify the ones that lie in between.

Example: All Merge Requests require two Maintainers to approve it before it can be merged. None of the approvals can come from the person initiating the MR.