RFE: List possible authentication method slugs in the README

[wdoekes]

'totp u2f yubikey'
(and possibly others)

[Urth]

The slug is constructed from the name used to register the authentication method with kleides-mfa.
kleides-mfa has default plugins for u2f/webauthn, otp_totp, otp_static and django-otp-yubikey which, when installed, are named: u2f, totp, recovery-code and yubikey respectively.