Add missing import to tutorial. Fixes #599

docs/tutorial/4-authentication-and-permissions.md

@@ -54,6 +54,8 @@ You might also want to create a few different users, to use for testing the API.
 
 Now that we've got some users to work with, we'd better add representations of those users to our API.  Creating a new serializer is easy:
 
+    from django.contrib.auth.models import User
+
     class UserSerializer(serializers.ModelSerializer):
         snippets = serializers.ManyPrimaryKeyRelatedField()
 
@@ -188,4 +190,4 @@ We've now got a fairly fine-grained set of permissions on our Web API, and end p
 
 In [part 5][tut-5] of the tutorial we'll look at how we can tie everything together by creating an HTML endpoint for our hightlighted snippets, and improve the cohesion of our API by using hyperlinking for the relationships within the system.
 
-[tut-5]: 5-relationships-and-hyperlinked-apis.md
+[tut-5]: 5-relationships-and-hyperlinked-apis.md
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