Custom serializer details

[blairj09]

Provide documentation and examples for building custom serializers.

[JosiahParry]

There still isn't any documentation as to how to create a custom serializer. If contributing to plumber it is fairly straightforward. It is not so as a plumber user.

There is no such documentation in the places one would think of looking:

• https://www.rplumber.io/reference/serializers.html
• https://www.rplumber.io/articles/rendering-output.html#serializers-1