Add documentation about how to override the `passkeys_authenticatable` for `Devise.add_module`

[tcannonfodder]

Previously in #13; I thought the module was not being defined inside the gem, but it's actually defined in: https://github.com/ruby-passkeys/devise-passkeys/blob/main/lib/devise/passkeys.rb#L26

Instead, the template repo overrides the module definition with its implementation-specific definitions, which is what we actually want to happen here; since each app will have to create its own set of controllers.

We should update the README to include documentation about how the step is needed (and why!); and use the code from the template repo as example code

[johalloran01]

Located in passkeys.rb line 48.
Template repo: https://github.com/ruby-passkeys/devise-passkeys-template/blob/main/app/models/user.rb