add missing m.room.encryption properties

event-schemas/examples/m.room.encryption

@@ -1,6 +1,8 @@
 {
   "content": {
-    "algorithm": "m.megolm.v1.aes-sha2"
+    "algorithm": "m.megolm.v1.aes-sha2",
+    "rotation_period_ms": 604800000,
+    "rotation_period_msgs": 100
   },
   "event_id": "$WLGTSEFJJKJ:localhost",
   "origin_server_ts": 1476648761524,

event-schemas/schema/m.room.encryption

@@ -10,6 +10,16 @@ properties:
         description: |-
           The encryption algorithm to be used to encrypt messages sent in this
           room. For example, ``m.megolm.v1.aes-sha2``.
+      rotation_period_ms:
+        type: integer
+        description: |-
+          How long the session should be used before changing it. ``604800000``
+          (a week) is the recommended default.
+      rotation_period_msgs:
+        type: integer
+        description: |-
+          How many messages should be sent before changing the session. ``100`` is the
+          recommended default.
     required:
       - algorithm
     type: object