Add downloads shield to readme

Add shield to show how popular Loop habits is.

README.md

@@ -3,6 +3,9 @@
   <a href="https://github.com/iSoron/uhabits/actions?query=workflow%3A%22Build+%26+Test%22">
     <img src="https://github.com/iSoron/uhabits/workflows/Build%20&%20Test/badge.svg" />
   </a>
+  <a href="https://www.appbrain.com/app/loop-habit-tracker/org.isoron.uhabits">
+    <img src="https://www.appbrain.com/shield/org.isoron.uhabits.svg">
+  </a>
   <a href="https://github.com/iSoron/uhabits/releases/latest">
     <img src="https://img.shields.io/github/v/release/iSoron/uhabits" />
   </a>