Add link to PR.

pep-0559.rst

@@ -37,7 +37,7 @@ The Python equivalent of the ``noop()`` function is exactly::
     def noop(*args, **kws):
         return None
 
-The C built-in implementation is available as a pull request.
+The C built-in implementation is available as a pull request [1]_.
 
 
 Rejected alternatives
@@ -79,6 +79,12 @@ Coghlin's Dialogs (edited for formatting):
     it and having it do nothing useful.
 
 
+References
+==========
+
+.. [1] https://github.com/python/cpython/pull/3480
+
+
 Copyright
 =========