Update README.md

[Poussinou]

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">]()

You can now eventually add the fdroid and/or f-droid tags that will appear on the right side of the repo.