Add link/instructions to install Flatpak version of gajim nightly (master branch)

There are different ways to propagate the nightly version of Gajim (flatpak). Buttons could be used (like on https://wiki.gnome.org/Apps/Builder) or install instructions (like on https://wiki.gnome.org/Apps/Builder/Downloads).

The required file is https://ftp.gajim.org/flatpak/appstream/org.gajim.Gajim.flatpakref