docs(README): added Play Store ratings badge

Author: ErikBjare

README.md

@@ -2,6 +2,7 @@ aw-android
 ==========
 
 [![GitHub Actions badge](https://github.com/ActivityWatch/aw-android/workflows/Build/badge.svg)](https://github.com/ActivityWatch/aw-android/actions)
+[![Play Store ratings](https://PlayBadges.pavi2410.me/badge/ratings?id=net.activitywatch.android&country=us)](https://play.google.com/store/apps/details?id=net.activitywatch.android)
 
 A very work-in-progress ActivityWatch app for Android.