spec notifications key on power level event and provide @room in example

event-schemas/examples/m.room.power_levels

@@ -14,7 +14,10 @@
     "users": {
       "@example:localhost": 100
     },
-    "users_default": 0
+    "users_default": 0,
+    "notifications": {
+      "room": 20
+    }
   },
   "state_key": "",
   "origin_server_ts": 1431961217939,

event-schemas/schema/m.room.power_levels

@@ -85,6 +85,14 @@ properties:
             ``user_id`` is mentioned in the ``users`` key. Defaults to 0 if
             unspecified.
         type: number
+      notifications:
+        additionalProperties:
+          type: number
+        description: |-
+            The power level requirements for specific notification types.
+            This is a mapping from ``key`` to power level for that notifications key.
+        title: Notification power level requirements
+        type: object
     type: object
   state_key:
     description: A zero-length string.