main: turn auto-merge for sync PRs back on and describe it in CONTRIBUTING#5039

ralfhandl wants to merge 1 commit intoOAI:mainOAI/OpenAPI-Specification:mainfrom

ralfhandl:main-enable-auto-merge-for-sync-prsralfhandl/OpenAPI-Specification:main-enable-auto-merge-for-sync-prsCopy head branch name to clipboard