A section on the implications of including worlds

[macovedj]

In #245 a user encountered a bump when trying to import functionality from wasi:http that I'd guess will occur again.

Not sure if this belongs in the subsection on worlds, since it could also be part of #240, or there could perhaps be a section that pertains to all guest languages, as it probably isn't unique to the rust experience.

The user included wasi:http/proxy and didn't realize that they would have to implement the interfaces exported by it, as they were really only after importing the functionality that that world provided, and had no intention of authoring a component that exported any of it's functionality. Ideally we would clearly inform users to reserve include usage for when they want to define a world that is a superset of another.