Document the JSON output schema

[delcenjo]

The README shows --json but does not document the result shape (source, target, exact/normalized counts and fractions, examples). A short schema section would help anyone scripting against it.

[ritvikmudgal]

Hi! I'd like to work on this issue. Could you please assign it to me?

[delcenjo]

Thanks, it is yours. The fields come from results_to_json in src/splitcheck/render.py: each result has source and target (the split names), target_size, exact_overlap and normalized_overlap (counts), exact_fraction and normalized_fraction (the leakage as a fraction of the target), and examples (up to a few leaking rows). A short "JSON output" section in the README listing each field with its type is exactly right. Ping me on the PR if anything is unclear.