Please update MULLVAD provider specific settings to call out the username/password config needed

[aj0413]

Mullvad is odd one out in that it requires an 'm' in the password field and just uses the account number in the username one for auth.

When I first started using this project years ago, I learned this via issue thread on the topic, but was surprised to see it still not documented anywhere in either this or the new config repo.

Can we get this doc'd for new users? Mullvad has grown to a large user base now and should be considered a first class citizen.