data: add a basic AppData file

Adding a very basic AppData [1] file, with only 2 screenshots for now.
Complying with this FreeDesktop.org standard is required if we want to
get included in some future app stores such as Fedora's.

Unfortunately I haven't been able to validate the XML data yet, because
of this compilation problem [2].

[1] http://people.freedesktop.org/~hughsient/appdata/
[2] https://github.com/hughsie/appdata-tools/issues/10