Add example of adding PG to macOS zsh PATH

What does this Merge Request do and why?

If you have never worked with macOS\zsh it might be confusing to figure out yourself how to add an environment variable. Having a basic example for a default macOS shell should greatly speed things up.

Merge Request checklist

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