Need to update Picotool building instructions

[lurch]

Whilst looking into raspberrypi/picotool#45 I realised that https://datasheets.raspberrypi.com/pico/getting-started-with-pico.pdf doesn't mention the fact that LIBUSB_ROOT needs to be set when building picotool on Windows. It is mentioned in https://github.com/raspberrypi/picotool/blob/master/README.md but I guess not everyone will think to check that.
Probably wouldn't hurt to also add more detailed "libusub install instructions" than just "While if you are building on Microsoft Windows you can download and install a Windows binary of libusb directly from the libusb.info site." ?

(IIRC this only became an additional requirement after the release of picotool v1.1.0 which includes the -f/-F option to force a RP2040 to reset over USB.)

[lurch]

This now shouldn't be done until after raspberrypi/picotool#52 has been fixed.