[Assetic] added a note about the need to remove a route

_assetic route needs to be removed when use_controller is set to
false.

Author: bamarni

cookbook/assetic/asset_management.rst

@@ -337,6 +337,10 @@ the following change in your ``config_dev.yml`` file:
             'use_controller' => false,
         ));
 
+.. note::
+
+You'll also have to remove the ```_assetic``` route in your ```app/config_dev.yml``` file.
+
 Next, since Symfony is no longer generating these assets for you, you'll
 need to dump them manually. To do so, run the following: