Create "Feature Graphic" for F-Droid client

Now that the F-Droid client supports feature graphics, we should create one for F-Droid itself.

The guidelines say:

Your feature graphic is a powerful tool to show off your creative assets and attract users.

...

Requirements

• JPEG or 24-bit PNG (no alpha)
• Dimensions: 1024px by 500px

...

Tips

• Don't include any copy or important visual information near the borders of the asset - specifically near the bottom third of the frame.
• Try to center align any logo/copy information in the vertical and horizontal center of the frame.
• If adding text, use large font sizes.
• Your graphic may be displayed alone without the app icon.

I think we have the advantage that we can go as abstract as we like. By definition, people who are viewing the App Details page for the F-Droid package will know what F-Droid does. So we don't need to have screenshots in the feature graphics like some of the other examples:

Here are some examples. They seem to fall into the "Very minimalist" or "Way over the top" approaches (from here):

Abstract

Detailed with screenshots

Other

This is neither abstract nor over detailed. Given they are about holidays, it is very nice and simple but still related to their app (not abstract):

Yes! this would be great to have. On a related note, I think we should use fastlane supply to do the screenshoting of F-Droid. It'll automatically generate them for all the languages in the emulator.

mentioned in issue fdroidclient#1100

Small suggestion for the feature graphic:

This would require asking some famous app authors to use their icons.

Thanks @ByteHamster, I appreciate your suggestion. Looks like a pretty good place to start to me (though I do immediately think of DropBox when I look at it ;)) I've also asked our UX designer @crwinfrey if she can weigh in with a design over the coming weeks.