Add README examples and document API

This PR also contains a small code change (supported by tests) to ensure that +deep_merge+, +join_data+, and +join_array_data+ always return the +lhs+ object. On top of providing consistent, expected behavior, it also made creating and verifying the usage examples easier.

@adelevie Mind taking a look? Happy for feedback on whether the explanations are clear, and for YARD tips (or better doc tips in general) if you have them.