Add f-droid link

Author: gades

README.md

@@ -16,6 +16,9 @@ Devices with Android versions 8.1 and above
 
 ## Download
 
+[<img src="https://fdroid.gitlab.io/artwork/badge/get-it-on.png"
+     alt="Get it on F-Droid"
+     height="80">](https://f-droid.org/en/packages/cash.p.terminal/)
 [<img src="https://play.google.com/intl/en_us/badges/images/generic/en-play-badge.png"
      alt="Get it on Google Play"
      height="80">](https://play.google.com/store/apps/details?id=cash.p.terminal)