Migration doc

[Tobi-De]

@Tobi-De fastapi-users-tortoise doesn't seem like a drop-in replacement.

from fastapi_users.db import TortoiseUserDatabase
ImportError: cannot import name 'TortoiseUserDatabase' from 'fastapi_users.db' (/usr/local/lib/python3.10/site-packages/fastapi_users/db/init.py)

Can you please add a migration documentation?

Originally posted by @ShipraShalini in fastapi-users/fastapi-users-db-tortoise#3 (comment)