add the default configuration in graduation criteria.

Author: jiahuif

keps/sig-cloud-provider/2436-controller-manager-leader-migration/README.md

@@ -319,6 +319,7 @@ unsetting the `--enable-migration-config` flag.
 ##### Alpha -> Beta Graduation
 
 Leader migration configuration is tested end-to-end on at least 2 cloud providers.
+The default migration configuration is implemented and tested.
 
 ##### Beta -> GA Graduation