Issue #25136: Add reference to 'xcode-select --install' to Mac README.
diff --git a/Mac/README b/Mac/README
index 04c02a3..fe49938 100644
--- a/Mac/README
+++ b/Mac/README
@@ -46,7 +46,10 @@
The optional argument specifies which OS X SDK should be used to perform the
build. This defaults to ``/Developer/SDKs/MacOSX.10.4u.sdk``. When building
on OS X 10.5 or later, you can specify ``/`` to use the installed system
- headers rather than an SDK.
+ 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.