Update platform instructions to include Chromebooks

[PeterJCLaw]

From mentoring in schools, it turns out that some schools have Chromebooks and these are the only devices which are able to use some features -- most importantly in one case USB ports.

Notably at one school this meant they wanted to use the main computers for developing the code, then transfer it (via Google Drive) to the Chromebook and from there to the USB stick.

To account for this we should include instructions for using Chromebook everywhere that we provide platform specific descriptions of things which might reasonably be done on a Chromebook. (So probably everything apart from the simulator)