README: use F-droid badge with surrounding space (commons-app#3979)

* README: use F-droid badge with surrounding space

The F-droid badge in Commons has a newer revision that doesn't have
the surrounding blank space thus resulting in it being rendered bigger
than the play store badge.

Link to the older version of the image with surrounding blank space
to ensure both badges are of the same size.

* REDAME: bring back alt text for F-droid badge

Re-add the unintentionally removed F-droid badge alt text.

Author: sivaraam

README.md

@@ -7,7 +7,7 @@ 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://upload.wikimedia.org/wikipedia/commons/9/96/%22Get_it_on_F-droid%22_Badge.png" alt="Get it on F-Droid" height="90"/></a>
+<img src="https://upload.wikimedia.org/wikipedia/commons/archive/9/96/20200131184248%21%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>