Add metadata for F-Droid
Refs #90.
This adds metadata required for publishing in F-Droid.
For the official F-Droid repository, it is strongly encouraged to add metadata in the application’s source repo [...]
Either fastlane or Triple-T structures are supported. Both tools are free & open. Since Fastlane has 10 x stars/forks of Triple-T on GitHub and supports iOS additionally to the Google Play Store, I chose that format.