Add 'version' field

proposals/2946-spaces-summary.md

@@ -225,9 +225,10 @@ Justifications for the request API shape are the same as before with one excepti
    the server should not return node information under `rooms` nor should it return _any state events in this room_. NB: state
    events which _point to_ this room should still be included.
 
-The response body remains unchanged from the client format. Servers are unable to verify the auth chain of the returned events
-as they are typically not joined to the rooms returned. Servers MUST NOT persist these events in any potential room DAG that may
-be created if the server were to join the room.
+The response body remains unchanged from the client format with the exception of a `version` field added to each room object under `rooms`.
+Servers MUST use this field to determine how to decode the events in `events`. If the room version is unknown, the server may ignore these
+events/rooms. Servers are unable to verify the auth chain of the returned events as they are typically not joined to the rooms returned.
+Servers MUST NOT persist these events in any potential room DAG that may be created if the server were to join the room.
 
 Sending server behaviour:
  - When walking the spaces graph, if the server is not joined to a given room, remember the `via` server names and the room ID.