Update README.md

Author: TheMuellenator

README.md

@@ -156,6 +156,9 @@
 * [Cloud Firestore Package for Flutter](https://pub.dartlang.org/packages/cloud_firestore)
 * [Troubleshooting Firebase & Flutter: Common Errors](https://blog.londonappbrewery.com/troubleshooting-firebase-x-flutter-a974b2645689)
 * [Showing a Spinner: The Modal Progress HUD](https://pub.dartlang.org/packages/modal_progress_hud)
+* [Firebase Documentation](https://firebase.google.com/docs/)
+
+