Documentation: Add "Extending Deptcharge" manual

[jynik]

I put quite a bit of elbowgrease into ensuring that Depthcharge can be "a tool for quickly building one-off tools". However, the current documentation does not adequately reflect this.

This issue tracker item is to track some ideas and progress on building out a guide on how to extend Depthcharge. I envision this including a good description the Operation.register() functionality, and a solid discussion of some of the class private data members that are leveraged. This also begs the question, which is "really private" vs "intended for use by folks extending Depthcharge at runtime"? -- which is something that should be tackled in a sane manner.