Add documentation for CPP plugin creation

[tanaya-mankad]

Currently the fan_spec example utilizes a set of Python extensions that augment the standard .h/.cpp generation with custom features. However, other than these examples, there is no documentation on how to write an extension, how to use the existing HeaderEntry and ImplementationEntry classes, or how to link them to the program.

Maybe the namespace can be an input to the extension class, instead of a string constant in the code.