appdata: Add release tag

The appdata validation is now more strict and has a
mandatory requirement of the "release" tag [1]

[1] https://github.com/hughsie/appstream-glib/pull/296
1 job for gnome-3-32 in 15 minutes and 42 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #263409
flatpak-documents

00:15:42