CESG guidance isn't specific enough to be useful for devs

[timgent]

https://github.com/jcavell/security-guide-for-developers/wiki/At-the-Home-Office#before-you-start
"make sure your computer adheres to CESG end user device guidance"

I think we will need to give some brief, clear, guidance about exactly what we expect from people. The CESG guidance doesn't give much guidance on how to do things, just what needs to be achieved