Improve ReadMe for grub4dos/grub4efi

[Steve Si]

title Linux Mint VDI\\VHD \nBoot Fixed VHD or VDI Linux disk image

set VTOY=/_ISO/LINUX/Ubuntu Mint.vdi_VTNORMAL.vtoy

# Path must NOT contain a device name (must start with / character) - all disks are searched to find the file
# Note if file name contains spaces, do NOT precede the space character with \ character

if exist @uefi chainloader (bd)/_ISO/e2b/grub/vdiskchain "vdisk=%VTOY%"  
if exist @uefi boot
kernel (bd)/_ISO/e2b/grub/ipxe.krn "vdisk=%VTOY%" 
initrd (bd)/_ISO/e2b/grub/vdiskchain
errorcheck off
boot