[1.8.x] Added admonition about reusable apps and AUTH_USER_PROFILE.
[1.8.x] Added admonition about reusable apps and AUTH_USER_PROFILE.

Backport of 566c9362363f4b65cc64a35f907c2d6c39d0289e from master

pquentin/django,h4r5h1t/django-hauthy,h4r5h1t/django-hauthy,pquentin/django,pquentin/django,h4r5h1t/django-hauthy,h4r5h1t/django-hauthy