Add AppStream metainfo file #17
Description
This is required on Flathub and is consumed by e.g. KDE Discover, GNOME Software, …
https://github.com/wjt/flathub/blob/io.github.fragglet.sdl-sopwith/io.github.fragglet.sdl_sopwith.metainfo.xml is my work-in-progress. It may need revising in light of #16.