COMPAT: Do not run xargs with -r on OSX

OSX xargs has no -r, but it still terminates
when xargs is empty, which was the whole point
of passing in that flag in the first place.

[ci skip]

doc/source/contributing.rst

@@ -529,6 +529,11 @@ run this command, though it will take longer::
 
    git diff master --name-only -- '*.py' | grep 'pandas/' | xargs -r flake8
 
+Note that on OSX, the ``-r`` flag is not available, so you have to omit it and
+run this slightly modified command::
+
+   git diff master --name-only -- '*.py' | grep 'pandas/' | xargs flake8
+
 Backwards Compatibility
 ~~~~~~~~~~~~~~~~~~~~~~~