Add OS X build badge [CI skip]

We have OS X builds in another repo. Show the badge here too, to help spot OS X build breaks sooner (for example, if might pass on Linux).

README.rst

@@ -7,8 +7,12 @@ Pillow is the "friendly" PIL fork by `Alex Clark and Contributors <https://githu
 
 .. image:: https://travis-ci.org/python-pillow/Pillow.svg?branch=master
    :target: https://travis-ci.org/python-pillow/Pillow
-   :alt: Travis CI build status
+   :alt: Travis CI build status (Linux)
 
+.. image:: https://travis-ci.org/python-pillow/pillow-wheels.svg?branch=latest
+   :target: https://travis-ci.org/python-pillow/pillow-wheels
+   :alt: Travis CI build status (OS X)
+
 .. image:: https://pypip.in/v/Pillow/badge.png
     :target: https://pypi.python.org/pypi/Pillow/
     :alt: Latest PyPI version