Correct documentation about mixin usage
Correct documentation about mixin usage

`Configuration` class needs to be last in the inheritance list, otherwise it's not possible to override Django's default settings. Tests are already correct, just the documentation is wrong.
cato-/django-configurations,gatherhealth/django-configurations,nangia/django-configurations,NextHub/django-configurations,jezdez/django-configurations,jazzband/django-configurations,blindroot/django-configurations,seenureddy/django-configurations,pombredanne/django-configurations,incuna/django-configurations,jazzband/django-configurations