Add Mac install instructions - Update README.md

[LouisAUTHIE]

Some environment variables to avoid errors like cblas.h unfound, pcre2.h unfound or gfortran unfound.

[github-actions]

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[LouisAUTHIE]

I have read the CLA Document and I hereby sign the CLA

[andrewdalpino]

Fantastic! Thank you @LouisAUTHIE