Document how to add a new writer

[minchinweb]

c.f. #2899

There was nothing in the existing documentation about how to actually implement a custom writer. This adds a little bit (of documentation) to that end.

Pull Request Checklist

Resolves: #issue-number-here

• [ n/a ] Ensured tests pass and (if applicable) updated functional test output
• Conformed to code style guidelines by running appropriate linting tools
• [ n/a ] Added tests for changed code
• Updated documentation for changed code

[avaris]

Thanks

[justinmayer]

Thanks to @minchinweb for the contribution and to @avaris for reviewing. 🏅