Add F-Droid badge

[offa]

LocalMaterialNotes is available on F-Droid now.

closes #72

[offa]

Is there any preference of badge order?

[maelchiotti]

Thank you very much for helping getting Local Material Notes on F-Droid! I had already begun some work locally to add the F-Droid badge, along with some README updates, see #163, so I'll close this - but again thank you for going the extra mile here.