How-to: create a CSM model

[Kelvinrr]

A how-to guide explaining CSM models and how to create a new model would be very helpful.

Some knowledge the reader should have by the end:

• What is a CSM model? Why is it different than an ALE driver?
• What is the CSM state string and how is it different than the ISD?
• What functions need to be written and why?

[AustinSanders]

This issue may have some crossover with some of the docs in #36. It may be worth linking / referencing or adding to the existing docs instead of adding more pages. Specifically this section and this notebook