[rs/zerolog] Document default logger level (issue #678)

Job description

Fix zerolog issue 678: https://github.com/rs/zerolog/issues/678 Clarify the README documentation for the default logger level and which standard logging calls appear on stderr. Keep the explanation accurate to current behavior and add a focused example or validation where appropriate. Deliverables: upstream PR linked to the issue, changed files summary, and validation notes. The PR body or a linked issue comment must mention this OpenJobs listing URL.