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

Author: jaraco

setup.rst

@@ -404,6 +404,9 @@ commands.
 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 `readme <https://github.com/python/cpython/blob/master/Mac/README>`_.
+
 .. _clang: https://clang.llvm.org/
 .. _ccache: https://ccache.samba.org/