Added instructions on adding shared folder

If the user is only using VirtualBox, they need instructions on how to add a shared folder.
I left the second field blank (where you put /My_OpenKIM_VM/) and it automatically filled it out without the slash with the base name of the folder (My_OpenKIM_VM).  After rebooting, the mount appeared at /media/sf_{stub} where {stub} is the item that was in the second field.  It is mounted as root and requires sudo to read and write to.  

Make sure you install the virtualbox guest additions before you do these steps and reboot for auto mount.