add appdata

Gnome Software included in recent Gnome release (and in Fedora 20+) need this file to be able to search in the distribution available software and propose useful information to the user.

More details about AppData files is available here: ​http://people.freedesktop.org/~hughsient/appdata/

As this is a freedesktop project, this should be slowly adapted by other DE (p.e. Kde apper) and other distributions.

Per Fedora Guidelines, this file will be soon mandatory for each desktop application (and without, user probably won't be able to find/install this application)

Of course this file "should" be translated to have a real value...
It should be possible to manage translation with gettext (but this seems really complex...), so the proposed file is very simple, with english and french only.

Perhaps, a note on http://gmusicbrowser.org/translations.html about this file (and the .desktop one) will help...