Add note for using authtoken with custom django user model and south

docs/api-guide/authentication.md

@@ -169,6 +169,8 @@ The `obtain_auth_token` view will return a JSON response when valid `username` a
 
 Note that the default `obtain_auth_token` view explicitly uses JSON requests and responses, rather than using default renderer and parser classes in your settings.  If you need a customized version of the `obtain_auth_token` view, you can do so by overriding the `ObtainAuthToken` view class, and using that in your url conf instead.
 
+**Note:** If you are using `rest_framework.authtoken` with a custom Django user model and South. you have to insert a `needed_by = (('authtoken', '0001_initial'),)` to your user migration.
+
 ## SessionAuthentication
 
 This authentication scheme uses Django's default session backend for authentication.  Session authentication is appropriate for AJAX clients that are running in the same session context as your website.