Use F-droid badge from Wikimedia Commons (commons-app#2896)

Fixes commons-app#2895.

The F-droid seems to be too slow recently due to unusual load
as a consequence of which the F-droid badge is not getting
loaded.

So, switch to using the F-droid badge from Wikimedia Commons for now.

Author: sivaraam

README.md

@@ -5,7 +5,7 @@ The Wikimedia Commons Android app allows users to upload pictures from their And
 Initially started by the Wikimedia Foundation, this app is now maintained by grantees and volunteers of the Wikimedia community. Anyone is welcome to improve it, just choose among the [open issues][3] and send us a pull request :-) 
 
 <a href="https://f-droid.org/repository/browse/?fdid=fr.free.nrw.commons" target="_blank">
-<img src="https://f-droid.org/badge/get-it-on.png" alt="Get it on F-Droid" height="90"/></a>
+<img src="https://upload.wikimedia.org/wikipedia/commons/9/96/%22Get_it_on_F-droid%22_Badge.png" alt="Get it on F-Droid" height="90"/></a>
 <a href="https://play.google.com/store/apps/details?id=fr.free.nrw.commons" target="_blank">
 <img src="https://play.google.com/intl/en_us/badges/images/generic/en-play-badge.png" alt="Get it on Google Play" height="90"/></a>