Add badge "Get it on F-Droid"

Beside the "Get it on Google Play" there should be a "Get it on F-Droid" badge, if you already offer this possibility

• https://briarproject.org/index.html
• https://briarproject.org/download.html

Here you can find the badge:

mentioned in merge request !4 (merged)

closed via commit 4494ccfe

closed via merge request !4 (merged)

assigned to @grote