Add docs on when reviews should be cleared (#583)

CONTRIBUTING.md

@@ -75,7 +75,13 @@ A PR is considered to be **ready to merge** when:
 
 * It has received two approvals from Approvers/Maintainers (at
   different companies).
-* Major feedback items are resolved.
+* Feedback has been addressed.
+* Any substantive changes to your PR will require that you clear any prior
+  Approval reviews, this includes changes resulting from other feedback. Unless
+  the approver explicitly stated that their approval will persist across
+  changes it should be assumed that the PR needs their review again. Other
+  project members (e.g. approvers, maintainers) can help with this if there are
+  any questions or if you forget to clear reviews.
 * It has been open for review for at least one working day. This gives
   people reasonable time to review.
 * Trivial change (typo, cosmetic, doc, etc.) doesn't have to wait for