Mention phone numbers

Author: samtstern

auth/README.md

@@ -7,7 +7,7 @@ code and promote best practices (both user experience and security) for
 authentication.
 
 A simple API is provided for drop-in user authentication which handles the flow
-of signing in users with email addresses and passwords, and federated identity
+of signing in users with email addresses and passwords, phone numbers, and federated identity
 providers such as Google Sign-In, and Facebook Login. It is built on top of
 [Firebase Auth](https://firebase.google.com/docs/auth).