Need to document net-boot config options

[lurch]

Now that net-boot has moved to stable raspberrypi/rpi-eeprom#420 we probably ought to document the new config-options related to this.
There's a link in raspberrypi/rpi-eeprom#415 to a PDF which documents the new options.

@peterharperuk / @timg236 Is that PDF up-to-date with all the necessary options?
(Hmmm, I guess it can't be, because it doesn't mention the IMAGER_REPO_URL option mentioned here ? )

[peterharperuk]

Hi - I'm litterally working on a commit for the documentation as we speak :)

[aallan]

Thanks @peterharperuk