Add 'realpath' dependency for macOS users

webchickrequested to merge
webchick/gitlab-development-kit:main into main

What does this Merge Request do and why?

Addresses #1250 (closed), by documented the need for macOS users to install realpath prior to running the one-time installation script.

Merge Request checklist

  • This change is backward compatible.
  • [N/A] Tests added for new functionality. If not, please raise Issue to follow-up.
  • Documentation added/updated, if needed.
  • [N/A] gdk doctor test added, if needed.
  • [N/A] Add the ~highlight label if this MR should be included in the CHANGELOG.md.

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