Specify requirement of origin_server_ts

Signed-off-by: Benjamin Saunders <ben.e.saunders@gmail.com>

event-schemas/schema/core-event-schema/room_event.yaml

@@ -1,6 +1,6 @@
 allOf:
 - $ref: event.yaml
-description: In addition to the Event fields, Room Events may have the following additional
+description: In addition to the Event fields, Room Events have the following additional
   fields.
 properties:
   event_id:
@@ -13,6 +13,10 @@ properties:
     description: Required. Contains the fully-qualified ID of the user who *sent*
       this event.
     type: string
+  origin_server_ts:
+    description: Required. Timestamp in milliseconds on originating homeserver
+      when this event was sent.
+    type: number
   unsigned:
     description: Contains optional extra information about the event.
     properties:
@@ -33,5 +37,6 @@ required:
 - event_id
 - room_id
 - sender
+- origin_server_ts
 title: Room Event
 type: object