Add virtualbox shared folders piece to handbook

[Johannes Lundberg]

Hi

https://www.freebsd.org/doc/handbook/virtualization-guest-virtualbox.html

I couldn't find anything in the handbook and various source on internet was
misleading.
The magic recipe that worked for me was

# kldload vboxvfs
# mount_vboxvfs -w Shared /mnt   (for R/W mount of a shared folder name
"Shared" in virtualbox settings)

It would be nice if the handbook could be updated with this.

Although the handbook claims copy/paste is working I could not get it to
work with macos host and freebsd12 guest.