Add how to add a new document format to readme
Add how to add a new document format to readme

This commit adds a step by step guide for adding a new document format
along with how to to test that document format.

ministryofjustice/specialist-publisher,alphagov/manuals-publisher,alphagov/specialist-publisher,ministryofjustice/specialist-publisher,alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher,ministryofjustice/specialist-publisher,ministryofjustice/specialist-publisher,alphagov/specialist-publisher,alphagov/manuals-publisher,alphagov/manuals-publisher,alphagov/specialist-publisher-rebuild