Add link.

Doc/whatsnew/3.1.rst

@@ -88,7 +88,7 @@ order.  The :mod:`collections` module's :meth:`namedtuple._asdict` method now
 returns an ordered dictionary with the values appearing in the same order as
 the underlying tuple indicies.  The :mod:`json` module is being built-out with
 an *object_pairs_hook* to allow OrderedDicts to be built by the decoder.
-Support was also added for third-party tools like PyYAML.
+Support was also added for third-party tools like `PyYAML <http://pyyaml.org/>`_.
 
 .. seealso::