Add guidance for CBOR converter source generation

[peteroupc]

The interfaces ICBORConverter<T> and ICBORToFromConverter<T> offer a high level of customization on how CBOR objects can be serialized to and from .NET objects. Given that source generators allow an application to avoid reflection and enable self-package applications, add one or more documents giving guidance on generating source code for classes that implement ICBORConverter<T> and ICBORToFromConverter<T>. Such guidance is useful even for the current version of the CBOR library. For an example, see the JSON case.

I am aware of this technique being used in a third-party library.