add migration steps and documentation to move to django-allauth

[dicato]

django-allauth provides a more comprehensive solution to User registration and authentication than this project. Given the complexity of integrating different registration, authentication (two-factor, social auth, etc) with User management it generally makes sense to adopt one complete solution instead of combining many disparate solutions.

Ideally, django-registration should provide both code and documentation to move a Django project to django-allauth in a tested, repeatable way.