message: Clarification on README.old.md
diff: # Archived v3.12.1 documentation
  -THIS IS SAVED FOR POSTERITY. IT IS LIKELY OUT OF DATE.
  +THIS DOCUMENTATION IS FOR v3.12.1.
  -PLEASE VISIT https://react-querybuilder.js.org/ FOR THE LATEST DOCUMENTATION.
  +PLEASE VISIT https://react-querybuilder.js.org/ FOR THE LATEST DOCUMENTATION (v4+).
  ## Usage
  