Add full required imports to Generating Tokens example

Previously we were missing User and post_save.

docs/api-guide/authentication.md

@@ -162,6 +162,8 @@ The `curl` command line tool may be useful for testing token authenticated APIs.
 
 If you want every user to have an automatically generated Token, you can simply catch the User's `post_save` signal.
 
+    from django.contrib.auth.models import User
+    from django.db.models.signals import post_save
     from django.dispatch import receiver
     from rest_framework.authtoken.models import Token