Add FDroid metadata

[EmmanuelMess]

While being on it: would you mind placing your screenshots into fastlane file structure, so they show up on F-Droid as well? Same way you could btw manage most of your app's metadata, and deploy them to Google Play at the same time.

Originally posted by @IzzySoft in https://github.com/EmmanuelMess/Simple-Accounting/issues/137#issuecomment-540718648