Update authentication documentation in README (#2792)
Update authentication documentation in README (#2792)

* Update authentication documentation in README

See #2790

* Lint

Co-authored-by: Bryan Kendall <aee22ddf77d91edcd2a29133709a167f3a4e804e@google.com>
firebase/firebase-tools,firebase/firebase-tools,firebase/firebase-tools,firebase/firebase-tools,firebase/firebase-tools