Add some compatibility hacks.

proposals/2174-move-redacts-key.md

@@ -7,3 +7,11 @@ ID of the event being redacted.
 The presence of this field at the event level, rather than under the `content`
 key,  is anomalous. This MSC proposes that, in a future room version, the
 `redacts` property be moved under the `content` key.
+
+For backwards-compatibility with *older* clients, servers should add a `redacts`
+property to the top level of `m.room.redaction` events in *newer* room versions
+when serving such events over the Client-Server API.
+
+For improved compatibility with *newer* clients, servers should add a `redacts`
+property to the `content` of `m.room.redaction` events in *older* room versions
+when serving such events over the Client-Server API.