Add how to migrate to python-social-auth to the docs

[kaleissin]

You say that django-social-auth is deprecated in favor of python-social-auth, but you don't say how to migrate. For instance, will tables need to be renamed? To what? With South or by hand? A page in the django-social-auth documentation showing what needs to change in an existing project already using django-social-auth would be much appreciated.

[omab]

@kaleissin it's already there http://psa.matiasaguirre.net/docs/configuration/porting_from_dsa.html