acknowledge Nick Coghlan, add information on PyPy's pairtype

pep-0443.txt

@@ -213,6 +213,11 @@ only difference is whether the custom implementation is associated more
 closely with the data (object-oriented methods) or the algorithm
 (single-dispatch overloading).
 
+PyPy's RPython offers ``extendabletype`` [#pairtype]_, a metaclass which
+enables classes to be externally extended. In combination with
+``pairtype()`` and ``pair()`` factories, this offers a form of
+single-dispatch generics.
+
 
 Acknowledgements
 ================
@@ -222,7 +227,8 @@ PEAK-Rules, influences include Paul Moore's original issue
 [#issue-5135]_ that proposed exposing ``pkgutil.simplegeneric`` as part
 of the ``functools`` API, Guido van Rossum's article on multimethods
 [#artima2005]_, and discussions with Raymond Hettinger on a general
-pprint rewrite.
+pprint rewrite. Huge thanks to Nick Coghlan for encouraging me to create
+this PEP and providing initial feedback.
 
 
 References
@@ -248,6 +254,9 @@ References
 .. [#gnosis-multimethods]
    http://gnosis.cx/publish/programming/charming_python_b12.html
 
+.. [#pairtype]
+   https://bitbucket.org/pypy/pypy/raw/default/rpython/tool/pairtype.py
+
 
 Copyright
 =========