[23153] Add appdata description file to be used in Gnome's App store

See http://people.freedesktop.org/~hughsient/appdata/ for more details.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/branches/2.4@23154 57a11ea4-9604-0410-9ed3-97b8803252fd