Talk about appstream screenshots in docs

[btkostner]

We need to talk about how developers should take screenshots for their apps. Some talking points include:

• Only capture the app window. No desktop screenshots.
• A decent size for appcenter. (Not sure if we are still settling on an official size)
• Use only the one true GTK theme.

We also need to talk about how they should link the screenshots. I'm currently thinking of an auto fill way to resolve relative paths from the appstream data file to a github raw URL, but it's not finished yet.

[danirabbit]

These things are now mentioned in publishing requirements, but perhaps we should add a HIG section like we have for desktop files? Or maybe we should break "Metadata" into its own whole page to go into more detail on them