Update instructions for private repo workflow
Update instructions for private repo workflow

The instructions tell us how to make a repo private, but this means the
public cannot access our code.  The new method allows us to keep the
main codebase public, allow non-sensitive changes to be made in public
but keep sensitive changes hidden until they are ready to release.

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs