Add hint in chapter setup-file-server

[michaelkohls]

The command net conf addshare $SCHOOL /srv/samba/schools/$SCHOOL/ is only necessary while fresh install of LML with fileserver. See here:
https://ask.linuxmuster.net/t/lmn-7-3-fileserver-nachtraeglich-einbinden/11822/4
A hint should be added.