Add examples for shared folders.

[pwdonald]

This could be tricky as it seems that mounting a directory using VVFAT in qemu results in a segmentation fault if the ChromeOS host makes any changes to the shared directory.

[pwdonald]

After a lot of trial and error it hit me--just use a SFTP mount in the native ChromeOS Files App.
Port 10022 is already mapped in start_persist.sh using qemu hostfwd. Simply go to the Files app and add click 'Add new services' -> 'Secure Shell' then ssh as usual (you can set the mount point at the bottom of this dialog) which will then show up in the sidebar of the Files app as if it were mounted.

I will update the documentation with further details at a later time but for now anyone who stumbles on this I have tried the following with no success:

• Mapping a host directory as a drive in qemu.
  • When the guest tried to write back to the directory it would crash with a segmentation fault.
• Using Samba/NFS with hostfwd port mapping.
  • The Chrome NFS app did not like this. Apparently QEMU uses really high ports and Samba nor NFS would work. I think NFS might work if you use the insecure flag but I wasn't going that route.
• Tunneling Samba/NFS through SSH
  • It got messy, never seemed to work.

So in the end the easiest solution was right in front of my face. SFTP via Secure Shell.