Add the mapping from HCL -> JSON to the README?

The mapping from HCL to JSON (and vice versa) isn't immediately clear. I haven't been able to find concrete documentation for this yet. The fixtures help, but there seems to be a mix of valid/invalid examples.

Start by reviewing the README and the existing fixtures, which the issue identifies as the available context for HCL-to-JSON and JSON-to-HCL behavior. Compare the valid and invalid examples, then document the mapping clearly in the README, including enough distinction that readers can understand which examples are valid.

Written by the indexing model from the issue text.