Docs: Explain not using social_auth

[aaaaalbert]

As far as I can tell, it doesn't suffice to not install the social_auth module if you don't want it. You also have to remove it from the INSTALLED_APPS tuple in website/settings.py (at least), and I'm not sure if that suffices to remove the clearinghouse code base's dependency on it completely.

It's fine if it turns out that we absolutely need the module (or openid or oauth2 or whatever) for even the CH-intrinsic authentication pipeline to work. However, we need to know which exactly.