Write a document explaining the "inheritance" of Ion Schema

[popematt]

Many Ion Schema users are used to Java, JavaScript, and Python where a subclass/inheritor extends (or adds to) a base class. Ion Schema is the opposite—an "inheritor" is always narrower than (or equal to) it's base type. It doesn't always look this way to new users—the fields constraint in particular looks like it should be additive to people who are primarily familiar with Java.

This document should include recommendations for modeling scenarios such as:

• Enums
• Sealed variants of a type (i.e. sealed classes in Kotlin, enums in Rust)
• Abstract base classes
• Interfaces