message: Add note about 0.25.0 upgrade in UPDATING.md
diff: 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)
  