Update UPGRADE-4.0.md for namespace changes

Updated the upgrade documentation to reflect changes in namespace for Doctrine mapped super classes.

Author: Guite

UPGRADE-4.0.md

@@ -4,3 +4,7 @@ UPGRADE FROM 3.x to 4.0
 ## Remove MongoDB support
 
 MongoDB is not supported by this bundle anymore. All classes related to `doctrine/mongodb-odm` were removed.
+
+## Updated namespace for Doctrine mapped super classes.
+
+Your entities should now extend `Nucleos\UserBundle\Entity\BaseGroup` and `Nucleos\UserBundle\Entity\BaseUser` instead of `Nucleos\UserBundle\Model\Group` and `Nucleos\UserBundle\Model\User`.