Add install instructions for Linux

[vickytnz]

Add install instructions for linux

What is the issue?

The GOV.UK Prototype Kit has an install guide for Linux. https://prototype-kit.service.gov.uk/docs/install/requirements-linux
This kit does not.

There are a few options for this:

1. copy the Mac guide and rework it to have the few linux changes needed (mainly to do with the terminal and the npm package)
2. make the Mac and Linux combined instructions
3. make a holding page for Linux