xdg-app-builder: Update docs
xdg-app-builder: Update docs

alexlarsson/xdg-app,alexlarsson/xdg-app,matthiasclasen/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,amigadave/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,matthiasclasen/xdg-app,matthiasclasen/flatpak,GeorgesStavracas/flatpak,amigadave/flatpak,flatpak/flatpak,alexlarsson/xdg-app,amigadave/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/xdg-app,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/xdg-app,matthiasclasen/xdg-app,flatpak/flatpak,amigadave/flatpak,alexlarsson/xdg-app,matthiasclasen/flatpak,handsome-feng/flatpak