Add section about re-orgs

sync/optimistic.md

@@ -57,6 +57,16 @@ unknown). Specifically, `SYNCING` responses should be fork-specific; the search
 for a block on one chain MUST NOT trigger a `SYNCING` response for another
 chain.
 
+### Re-Orgs
+
+The consensus engine MUST support any chain reorganisation which does *not*
+affect the justified checkpoint. The consensus engine MAY support re-orgs
+beyond the justified checkpoint.
+
+If the justified checkpoint transitions from `SYNCING` -> `INVALID`, a
+consensus engine MAY choose to alert the user and force the application to
+exit.
+
 ## Merge Transition
 
 To protect against attacks during the transition from empty `ExecutionPayload`