Add screenshots to F-Droid

[aha999]

There is currently no screenshots when you open PassAndroid page on F-Droid, so we can't expect how the app looks like.

Please add screenshots to F-Droid.

[soshial]

I agree, that would be a nice thing to do.

[eighthave]

@aha999 @soshial or anyone else: if you can provide some good screenshots, I'll make a pull request here to add them via Fastlane metadata.