Update README.md

Author: TheMuellenator

README.md

@@ -155,6 +155,7 @@
 * [Firebase Auth Package for Flutter](https://pub.dartlang.org/packages/firebase_auth)
 * [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)