Update README.md

Author: TheMuellenator

README.md

@@ -150,6 +150,7 @@
 * [Pre-packaged Animations: Sprung](https://pub.dartlang.org/packages/sprung)
 * [Pre-packaged Animations: Animated Text Kit](https://pub.dartlang.org/packages/animated_text_kit)
 * [Refactoring Challenge Instructions](https://github.com/londonappbrewery/Flash-Chat-Flutter-Refactor-Challenge)
+* [Firebase Homepage](https://firebase.google.com/)