Add appdata file

This is based on FreeDesktop's [AppStream](http://www.freedesktop.org/wiki/Distributions/AppStream/) specification. It's a metadata file used by many Software Centers on Linux distributions. Ubuntu and OpenSUSE already support this. Fedora will support it at the next release, through Gnome's [Software Center](https://wiki.gnome.org/Design/Apps/Software).

See here for more:
http://people.freedesktop.org/~hughsient/appdata/

Ideally this file should also have links to screenshots. If you upload a couple of them at sparkleshare.org I could add them to the file.