Update README.md

Hi, This small PR adds a button to get your app on F-Droid. You can later add a button for Google Play the same way : `[<img src="https://play.google.com/intl/en_us/badges/images/generic/en-play-badge.png" alt="Get it on Google Play" height="80">]()` It also adds a link to get the latest APK from the Releases Section. You can now eventually add the [fdroid](https://github.com/topics/fdroid) and/or [f-droid](https://github.com/topics/f-droid) tags that will appear on the right side of the repo.