Document the DefaultBranch key
Document the DefaultBranch key

This can be set in .flatpakrepo files for a while.
It just wasn't documented.

GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,flatpak/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak