mention new tests and implied_indexes in docs

master/docs/developer/database.rst

@@ -1066,6 +1066,11 @@ Your upgrade script should have unit tests.  The classes in
 Unit test scripts should be named e.g.,
 :file:`test_db_migrate_versions_015_remove_bad_master_objectid.py`.
 
+The :file:`master/buildbot/test/integration/test_upgrade.py` also tests
+upgrades, and will confirm that the resulting database matches the model.  If
+you encounter implicit indexes on MySQL or Postgres, that do not appear on
+SQLite, add them to ``implied_indexes`` in :file:`master/buidlbot/db/model.py`.
+
 Database Compatibility Notes
 ----------------------------