doc: add app download badges to README

README.md

 
 ### Mobile apps
 
-You can get our mobile versions from here:
-
 * [Android](https://play.google.com/store/apps/details?id=org.jitsi.meet)
+
+[<img src="resources/img/google-play-badge.png" height="50">](https://play.google.com/store/apps/details?id=org.jitsi.meet)
+
+* [Android (F-Droid)](https://f-droid.org/en/packages/org.jitsi.meet/)
+
+[<img src="resources/img/f-droid-badge.png" height="50">](https://f-droid.org/en/packages/org.jitsi.meet/)
+
 * [iOS](https://itunes.apple.com/us/app/jitsi-meet/id1165103905)
 
+[<img src="resources/img/appstore-badge.png" height="50">](https://itunes.apple.com/us/app/jitsi-meet/id1165103905)
+
 You can also sign up for our open beta testing here:
 
 * [Android](https://play.google.com/apps/testing/org.jitsi.meet)