Add F-Droid badge

[PanderMusubi]

On https://opendocument.app/ please add an F-Droid badge with a link to https://f-droid.org/en/packages/at.tomtasche.reader/

For information on F-Droid badges, see https://gitlab.com/fdroid/artwork/tree/master/badge#designs

Unfortunately, at the moment we do not have 1-bit version (black and whilte).