WIP: Mention and link to the macOS readme in the repo. (#454)

* Mention and link to the macOS readme in the repo. Fixes #453.

* Rename hyperlink to 'macOS readme' to avoid conflict with Windows 'readme'

* Update link to macOS Readme

* Capitalize README.

Co-authored-by: Ezio Melotti <ezio.melotti@gmail.com>

Author: jaraco

setup.rst

@@ -418,6 +418,10 @@ just ask for assistance on the core-mentorship list.
 Explaining how to build optional dependencies on a UNIX based system without
 root access is beyond the scope of this guide.
 
+For more details on various options and considerations for building, refer
+to the `macOS README
+<https://github.com/python/cpython/blob/main/Mac/README.rst>`_.
+
 .. _clang: https://clang.llvm.org/
 .. _ccache: https://ccache.dev/