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.
Merge pull request #292 from alphagov/how-to-add-a-new-document-format

Add how to add a new document format to readme
Remove object-specific builders for parameter-specific builders

Many of the builders that inherit from `SpecialistDocumentBuilder`
tend to do the same thing which can be summarised as the following bit
of code:

```
def call(attrs)
  super(attrs.merge(document_type: "some_document_type_label"))
end
```

They're adding a `document_type` key to the `attrs` hash we pass in.

Rather than create a number of objects for the same task, instead
provide the difference as a parameter so that we can better understand
not only what's different about our builders, but also what's the
same.