Issue #25136: Add reference to 'xcode-select --install' to Mac README.

Mac/README

@@ -46,7 +46,12 @@ OS X specific arguments to configure
   The optional argument specifies which OS X SDK should be used to perform the
   build.  If xcodebuild is available and configured, this defaults to
   the Xcode default MacOS X SDK, otherwise ``/Developer/SDKs/MacOSX.10.4u.sdk``
-  if available or ``/`` if not.
+  if available or ``/`` if not.  When building on OS X 10.5 or later, you can
+  specify ``/`` to use the installed system headers rather than an SDK.  As of
+  OS X 10.9, you should install the optional system headers from the Command
+  Line Tools component using ``xcode-select``::
+
+     $ sudo xcode-select --install
 
   See the section _`Building and using a universal binary of Python on Mac OS X`
   for more information.