Indicate that this proposal requires a new room version

proposals/2403-knock.md

@@ -37,6 +37,9 @@ must be set to "knock" for a knock to succeed. This means that existing rooms
 will need to opt into allowing knocks in their rooms. Other than allowing
 knocks, "knock" is no different from the "invite" join rule.
 
+As we're updating the join rules, and thus the auth rules, this proposal will
+need to be introduced as part of a new room version.
+
 ## Membership changes
 Once someone has sent a `knock` membership into the room, the membership for
 that user can be transitioned to the following possible states:
@@ -237,6 +240,11 @@ If they deny, then a leave membership event is sent in the room, and the
 knocking user will be notified through existing flows (matching the behaviour
 of when an invite is recinded).
 
+TODO: Federation passing certain needed state events from the server that has
+the room to the server that's knocking. Can we filter the events sent over so
+that we can avoid the security issue with invites that we found earlier? (And
+then backport this filter to invites? :)
+
 
 ## Server-Server API
 Similarly to join and leave over federation, a ping-pong game with two new