As a 3rd party developer I would like documentation on how to add a trestle task to the codebase.

[butler54]

Issue description / feature objectives

Trestle tasks are the most likely entry point for an external developer to use trestle as in many ways they are effectively 'embedded scripts'.

Provide a recipe book for users (in the docs site) on how to contribute a task.

Caveats / Assumptions

• Interface is consistent.

Completion Criteria

• Docs site updated and deployed