CONTRIBUTING does not say how to add a COMMIT_EXCEPTIONS or UNLISTED_AUTHORS entry #1261

Where: CONTRIBUTING.md, scripts/check_contributors.py.

Current gap: The checks now excuse people through two dictionaries in the script (three contributors whose commit email is not linked, and one author whose commit cites a pull request that does not exist), but nothing explains when an entry is justified or how to add one.

Suggested approach: Add a short section to the checks table's contributors row or below it: when to use each list, the required reason, and a pointer to the Git identity map.

Done when: the section exists and names both dictionaries.