[1.7.x] Added missing docs for 1_7.W001 check.

Backport of 098368d82f from master

docs/ref/checks.txt

@@ -153,6 +153,13 @@ that might occur as a result of a version upgrade.
 
 * **1_6.W001**: Some project unit tests may not execute as expected.
 * **1_6.W002**: ``BooleanField`` does not have a default value.
+* **1_7.W001**:  Django 1.7 changed the global defaults for the
+  ``MIDDLEWARE_CLASSES.``
+  ``django.contrib.sessions.middleware.SessionMiddleware``,
+  ``django.contrib.auth.middleware.AuthenticationMiddleware``, and
+  ``django.contrib.messages.middleware.MessageMiddleware`` were removed from
+  the defaults. If your project needs these middleware then you should
+  configure this setting.
 
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