Add a .m.rule.tombstone default push rule for room upgrades

As per [MSC1930](https://github.com/matrix-org/matrix-doc/pull/1930)

There are no known changes to this proposal since it was accepted.

changelogs/client_server/newsfragments/2020.feature

@@ -0,0 +1 @@
+Add a ``.m.rule.tombstone`` default push rule for room ugprade notifications.

specification/modules/push.rst

@@ -369,6 +369,37 @@ Definition:
     }
 
 
+``.m.rule.tombstone``
+`````````````````````
+Matches any event whose type is ``m.room.tombstone``. This is intended
+to notify users of a room when it is upgraded, similar to what an
+``@room`` notification would accomplish.
+
+Definition:
+
+.. code:: json
+
+    {
+        "rule_id": ".m.rule.tombstone",
+        "default": true,
+        "enabled": true,
+        "conditions": [
+            {
+                "kind": "event_match",
+                "key": "type",
+                "pattern": "m.room.tombstone"
+            }
+        ],
+        "actions": [
+            "notify",
+            {
+                "set_tweak": "highlight",
+                "value": true
+            }
+        ]
+    }
+
+
 ``.m.rule.roomnotif``
 `````````````````````
 Matches any message whose content is unencrypted and contains the