Add note about 0.25.0 upgrade in UPDATING.md (apache#4883)

UPDATING.md

@@ -3,8 +3,13 @@
 This file documents any backwards-incompatible changes in Superset and
 assists people when migrating to a new version.
 
+## Superset 0.25.0
+Superset 0.25.0 contains a backwards incompatible database migration script.
+If you run a production system you should schedule downtime for this
+upgrade.
+* [4587](https://github.com/apache/incubator-superset/pull/4587)
+
 ## Superset 0.23.0
 
 * [4565](https://github.com/apache/incubator-superset/pull/4565)
-* [4587](https://github.com/apache/incubator-superset/pull/4587)
 * [4835](https://github.com/apache/incubator-superset/pull/4835)