Add screenshots for F-Droid (fastlane folder)

[jaller94]

This PR aims at adding screenshots to the listing in the F-Droid store. Descriptions/ images can either be added to this repo or to https://gitlab.com/fdroid/fdroiddata/tree/master/metadata

The files get updated to the F-Droid store on a release, not by merging this PR.

• [ ] A feature image can be added by adding the file ./fastlane/metadata/android/en-US/images/featureGraphic.png.

How to add screens to F-Droid: https://f-droid.org/en/docs/All_About_Descriptions_Graphics_and_Screenshots/

Fastlane's doc: https://docs.fastlane.tools/actions/supply/#images-and-screenshots