Fix typo in comment.

I have no idea why I made it a map[string]interface{} and it's a pain to deal
with.

Rename 'GoogleOpts' to 'ProviderOpts'.
Rename Principals to AdditionalPrincipals to match the config option.

I can never remember how markdown links work.

Include some notes on configuration

Add `use_tls` option and another `additional_principal`