Add a policy for handling retrospective vetos

For some time, we've been informally implementing a "revert early"
policy around handling -2 worthy complaints after a patch has been
merged. These reverts can be contentious, so it's appropriate we make
it very clear what the policy and procedure here is in order to lessen
the contentiousness of these reverts.

Co-authored-by: Michael Still <mikal@stillhq.com>

Change-Id: I90e4153a80c859a928630809aef31e7313f08906
Merge "Add a policy for handling retrospective vetos"