Add docs on when reviews should be cleared (#1556)

CONTRIBUTING.md

@@ -100,7 +100,13 @@ A PR is considered to be **ready to merge** when:
   different companies). This is not enforced through technical means
   and a PR may be **ready to merge** with a single approval if the change
   and its approach have been discussed and consensus reached.
-* Major feedbacks 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 changes (typo, cosmetic, doc, etc.) do not have to wait for