Add check for release notes update. (#582)
Add check for release notes update. (#582)

All PRs merging to `main` must either modify `release_build_files/readme.md` or contain the label `skip-release-notes`.
firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk