Stop referencing `require` as part of creating a new `*Builder`

As a result of #293 this is no longer true, so update the docs so that
#we stop confusing people who read docs before diving into code.
Merge pull request #294 from alphagov/update-README-to-stop-referencing-requires-when-creating-new-builders

Stop referencing `require` as part of creating a new `*Builder`