Document custom class implementation

[JasonWeill]

Problem

The operators documentation reads, in part,

For more information on how to write a custom implementation, please to [sic] our developer’s guide.

The developer's guide does not indicate how to write a custom scheduler, or how to write any other custom class.

Proposed Solution

Add documentation, with examples, about how to write a custom implementation of classes like the scheduler, environment manager, and execution manager.