Update FirebaseUI Auth documentation (#735)

Includes changes made in #690 for a privacy policy in the AuthUI sign-in intent builder.

Author: WillieCubed

auth/README.md

@@ -175,14 +175,15 @@ startActivityForResult(
     RC_SIGN_IN);
 ```
 
-If a terms of service URL and a custom theme are required:
+If a terms of service URL, privacy policy URL, and a custom theme are required:
 
 ```java
 startActivityForResult(
     AuthUI.getInstance()
         .createSignInIntentBuilder()
         .setProviders(...)
         .setTosUrl("https://superapp.example.com/terms-of-service.html")
+        .setPrivacyPolicyUrl("https://superapp.example.com/privacy-policy.html")
         .setTheme(R.style.SuperAppTheme)
         .build(),
     RC_SIGN_IN);