Make Shared Folder accessible by default in at least certain Linux variants

[l-w-c]

As explained ​here, it takes a running special command line in order to be able to enter shared folders instead of only getting "permission denied" messages.

It's explained there it might be different for each flavor of Linux, but how about giving solutions to at least popular packages like Ubuntu and Lubuntu?

As seen ​here in Lubunto the command (that also saved the day for me) is:

sudo usermod -a -G vboxsf $(whoami)

If not, then maybe at least produce a message after installing Guest Additions in Linux to invite the user to look up how to make shared folders work.