community/flatpak: add post-install message about XDG directories
I took me a long time to figure out what is wrong, why I don't have access to ~/Downloads directory from Flatpacked Firefox on Sway desktop.
xdg-user-dirs-update just creates config file
$XDG_CONFIG_HOME/user-dirs.dirs
that defines location of each
well-known XDG directory and also creates these directories.
It's a one-time thing, this config could be just copied from a
template... That's why I'm not proposing to add it as a dependency
of flatpak.