Setup guide should link to platform readme (macOS)

[jaraco]

Today I was reading the devguide to try to understand how framework builds work, but found it inadequate. I later learned that the Mac/README file in the repo has more detail. The devguide should reference that in setup/#macos. Filing this ticket so I don't forget.

[Mariatta]

GitMate.io thinks possibly related issues are python/core-workflow#30 (Link to developer guide in CPython repo description), #230 (Add a reference link to the "Python Extension Patterns" guide?), python/core-workflow#224 (Double link to the same PR), #124 (Github organization link is secret and not accessible from core-dev guide), and #221 (broken links and 404).

[terryjreedy]

Ned Deily has acknowledged that the current information about building Python on Mac is obsolete and inadequate. (For instance, tcl/tk is now included, and only 64 bit should be built for 3.7+.) But he has not yet updated it. Before linking the readme, I think he should be asked if the more detail is accurate.