Add explicit statement about how NHS Frontend relates to GOV.UK Frontend

[vickytnz]

Bug Report

What is the issue?

It's not clear how NHS Frontend relates to GOV.UK Frontend - how much is it inspired by GOV.UK Frontend, can people expect that everything in GOV.UK Frontend will get ported over to NHS Frontend in a particular amount of time, or is there an implicit understanding that teams may end up doing a lot of localised copy-paste from that system and eventually build it in to NHS Frontend? At present there are a lot of missing components and styles.

This is apparently when comparing the GOV.UK Design System component list https://design-system.service.gov.uk/components/ and the NHS Design System https://service-manual.nhs.uk/design-system/components