Added instructions on adding shared folder

If the user is only using VirtualBox, they need instructions on how to add a shared folder.
1) See command below:

2) Well, I think to do so you would need to not use automount.  You can then mount it using 

```
sudo apt-get install virtualbox-guest-utils
mkdir /path/to/mount
mount -t vboxsf SHARENAME /path/to/mount -o rw,uid=1000,gid=1000
```

3) See above, also listed in the GUI install section.