Add some references.

pep-3156.txt

@@ -925,6 +925,26 @@ Open Issues
   https://github.com/mnot/thor/blob/master/doc/events.md for examples.)
 
 
+References
+==========
+
+- PEP 380 describes the semantics of ``yield from``.  TBD: Greg
+  Ewing's tutorial.
+
+- PEP 3148 describes ``concurrent.futures.Future``.
+
+- PEP 3153, while rejected, has a good write-up explaining the need
+  to separate transports and protocols.
+
+- Nick Coghlan wrote a nice blog post with some background, thoughts
+  about different approaches to async I/O, gevent, and how to use
+  futures with constructs like ``while``, ``for`` and ``with``:
+  http://python-notes.boredomandlaziness.org/en/latest/pep_ideas/async_programming.html
+
+- TBD: references to the relevant parts of Twisted, Tornado, ZeroMQ,
+  pyftpdlib, libevent, libev, pyev, libuv, wattle, and so on.
+
+
 Acknowledgments
 ===============