Update README to include notes on EFI Graphics Resolution command

[michaelreid]

The script got me up and running in no time with a new El Capitan guest on my Sierra host.

But I struggled with resolution until I came across this command from some online video comments. It was in the VirtualBox documentation and this was the only thing that worked for me.

The extradata for my host now looks like:

Key: GUI/Fullscreen, Value: true
Key: GUI/HiDPI/UnscaledOutput, Value: true
Key: GUI/LastCloseAction, Value: PowerOff
Key: GUI/LastNormalWindowPosition, Value: 139,44,1024,789
Key: VBoxInternal2/EfiGopMode, Value: 4
Key: VBoxInternal2/EfiGraphicsResolution, Value: 1440x900

Thanks!

[marco-m]

@geerlingguy did you have a chance to look at this PR ?

[geerlingguy]

I haven't personally had this issue, but it may be worth including the extra documentation for those that do.