deflogin: added sudoers security note

/etc/sudoers is persistent with regard to userdel(8)
so removing a LiveCD user isn't going to drop this kind
of the added privilege and might result in an unintended
grant of those by adding a user with the same name after
permanent LiveCD installation.

This has been spotted by Speccyfighter:
https://bugzilla.altlinux.org/31071

features.in/deflogin/rootfs/image-scripts.d/50-users

@@ -18,6 +18,7 @@ set_password() { echo "$1:$2" | chpasswd; }
 
 set_admin() { usermod -a --groups "wheel" "$1"; }
 
+# NB: one must care to purge this from LiveCD if it's installed permanently
 set_sudo() {
 	[ ! -w "/etc/sudoers" ] ||
 		echo "$1	ALL=(ALL) ALL" >> "/etc/sudoers"