Add 'realpath' dependency for macOS users

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.