Improve documentation to ease users' transition to this gem

[nicolas-besnard]

Would you accept a PR to:

• Improve the rodauth:migrations to add a new parameter (--account-name?) to easily create new model / migration when using rodauth with multi configuration
• Add documentation on how to create new account programmatically
• Add documentation about the new RodauthController to create when using multiple accounts

[janko]

1. Yes, that sounds like a good idea 👍 I would maybe name the option just --name.
2. There is already an official guide for this. Maybe we can link to it from the README (e.g. it would be useful in the testing section).
3. Yes, makes total sense that a new configuration would require a different controller (e.g. for rendering different views) 👍

[janko]

I will close this one and await the pull requests 😉