README doc review for root, cmd, and examples

[nick-stroud]

Submission Checklist

• Have you installed and run this change against pre-commit? (pre-commit install)
• Are all tests passing? (make tests)
• Have you written unit tests to cover this change?
• Is unit test coverage still above 80%?
• Have you updated all applicable documentation?
• Have you followed the guidelines in our Contributing document?

[heyealex]

Are these bullets no longer valid?

[nick-stroud]

I have been discussing with @tpdownes. I have added back reference to the packages as it is the best cover all in our current state. I think long term we should find a way to update our find and install commands to be compatible with both GNU and BSD and remove this requirement.

[tpdownes]

I agree with the take and the final text which is trying to find a faster path to compiling ghpc on macOS. The bottleneck being installation of XCode and Command Line Tools.