Only show README content warning when README will display
Only show README content warning when README will display

We don't want to refer to 'the README below' and then just show
an error message explaining why the README isn't appearing.
We should only show the content warning when it's relevant.

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