Add AppData file

AppData is a new specification similar to .desktop for package/software metadata. It intends to contain much more metadata then a .desktop file including screenshots, a description, etc. Fedora is quite a large distribution, and they're switching to gnome-software for GUI package management. Having an appdata file will help provide support to something that is trying to get off the ground, and Gnome-Software will provide preference to packages with AppData. As of Fedora 22, an AppData file will be required to show up in Gnome-Software.