Add changelog entry for m.tag ordering

Signed-off-by: Travis Ralston <travpc@gmail.com>

changelogs/client_server.rst

@@ -19,6 +19,8 @@ Unreleased changes
     (`#1106 <https://github.com/matrix-org/matrix-doc/pull/1106>`_).
   - Clarify default values for some fields on the /search API
     (`#1109 <https://github.com/matrix-org/matrix-doc/pull/1109>`_).
+  - Clarify that ``m.tag`` ordering is done with numbers, not strings
+    (`#1139 <https://github.com/matrix-org/matrix-doc/pull/1139>`_).
 
 - Changes to the API which will be backwards-compatible for clients: