Update README.md with links to IzzyOnDroid

[Poussinou]

Hi,

This small PR adds a button to get your app via the IzzyOnDroid open source app repo in a F-Droid client, as well as a badge at the top of the README.md

It will help people without Google Play to find and use your app, as well as helping them to keep it updated.

Never forget to attach your apk to the release when tagging a new version, so that Izzy can take it and make it available via a F-Droid client to your users ;)