Document that container aware migrations are not special anymore

Author: goetas

UPGRADE.md

@@ -72,3 +72,6 @@ doctrine_migrations:
 Upgrading this bundle to `3.0` will also update the `doctrine/migrations` library to the version `3.0`.
 Backward incompatible changes in `doctrine/migrations` 3.0 
 are documented in the dedicated [UPGRADE](https://github.com/doctrine/migrations/blob/3.0.x/UPGRADE.md) document. 
+
+- The container is not automatically injected anymore when a migration implements `ContainerAwareInterface`. Custom
+migration factories should be used to inject additional dependencies into migrations.