update readme instructions on publishing-api user
update readme instructions on publishing-api user

Deleted instructions on creating empty user, as that is covered by the paragraph above using db:seed task
alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher,alphagov/specialist-publisher,alphagov/specialist-publisher,alphagov/specialist-publisher-rebuild