Add cookbook documentation demonstrating safe multi-threaded usage of IonValue

[tgregg]

Safely accessing an IonValue concurrently requires calling makeReadOnly in a single thread before initiating concurrent access. Violating this contract can result in non-deterministic behavior that is hard to root cause. Although the API documentation makes this contract clear, adding documentation that introduces the concept to new users early could reduce the risk that it is violated.

[tgregg]

We could also create a Discussion that would allow users to provide feedback on the usability of ion-java in multi-threaded contexts.