add documentation for model and implementation notes

[jbphet]

We generally have two standard documents: model.md that describes how the model works (often used for sharing with users if they ask about how the sim works) and implementation-notes.md (used primarily by developers that need to understand the code). Theses documents should be added. The previous Java documents can probably be leveraged for this.

[jbphet]

I transferred the documentation from the Java simulation and updated it for the JavaScript version. I think this should be sufficient. Closing.