Add UNAUTHENTICATED_USER = None note (#5679)
Add UNAUTHENTICATED_USER = None note (#5679)

When removing authentication entirely you cannot import `django.contrib.auth.models.AnonymousUser`

Closes #3494
tomchristie/django-rest-framework,kgeorgy/django-rest-framework,tomchristie/django-rest-framework,kgeorgy/django-rest-framework,tomchristie/django-rest-framework,kgeorgy/django-rest-framework