Add information about using Notify with the NHS.UK prototype kit

[vickytnz]

The GOV.UK prototype kit has instructions about using Notify. https://prototype-kit.service.gov.uk/docs/using-notify

We cannot use NHS.UK Notify for prototypes (as it is not free) but may be able to use the GOV.UK version with NHS branding.

This needs more investigation to understand what is possible.

[vickytnz]

Notes that we should use GOV.UK Notify (and not NHS Notify).

By setting the organisation it will also automatically create the branding for the NHS email.