api-guide/views.md - add imports to code example

* It wasn't clear where `Response` should be imported from.

docs/api-guide/views.md

@@ -19,6 +19,9 @@ Using the `APIView` class is pretty much the same as using a regular `View` clas
 
 For example:
 
+    from rest_framework.views import APIView, Response
+    from rest_framework import authentication, permissions
+
     class ListUsers(APIView):
         """
         View to list all users in the system.