Add guide for creating modules.

[ta0kira]

Specifically:

• .zeolite-module semantics and formatting.
• C++ extensions, once Streamline C++ extensions. #50 is done.

[ta0kira]

Specifically for C++ modules:

• How to call functions.
• How to use self, Var_self, parent, and parent->parent.
• Prohibition of @type data members.

[ta0kira]

C++ modules are probably better documented in example so that the code can be tested. For example, create an extension and outline the steps in a readme, without being verbose about intermediate file states, then document things like function calls directly in the C++ code.

[ta0kira]

The C++ example should also cover TRACE_CREATION and CAPTURE_CREATION.