added missing information in UPDATE file

UPDATE.md

@@ -12,6 +12,10 @@ RC4 to RC5
 * The `exception_controller` setting has been moved from the `framework`
   section to the `twig` one.
 
+* The custom error pages must now reference `TwigBundle` instead of
+  `FrameworkBundle` (see
+  http://symfony.com/doc/2.0/cookbook/controller/error_pages.html)
+
 * `EntityUserProvider` class has been moved and FQCN changed from
   `Symfony\Component\Security\Core\User\EntityUserProvider` to
   `Symfony\Bridge\Doctrine\Security\User\EntityUserProvider`.