Perhaps not full documented: db layer for multitentant

In the documentation https://github.com/citusdata/django-multitenant#automating-composite-foreign-keys-at-db-layer it's said that the `django.backends.postgresql could be replaced with `django_multitenant.backends.postgresql` in `ENGINE` in `settings.py.

But when I do this and run the server, I get the error:

```
django.core.exceptions.ImproperlyConfigured: 'django_multitenant.db.backends.postgresql' isn't an available database backend or couldn't be imported. Check the above exception. To use one of the built-in backends, use 'django.db.backends.XXX', where XXX is one of: 'mysql', 'oracle', 'postgresql', 'sqlite3'`
````

Perhaps it's missing an import, but it's not clear which import I should make. So I think it should be explained in the documentation as well?