Add source link for asyncio.

Guido van Rossum · Guido van Rossum · commit f8d0ff9f423d · 2013-11-22T16:53:25.000-08:00
diff --git a/Doc/library/asyncio.rst b/Doc/library/asyncio.rst
@@ -6,6 +6,9 @@
 
 .. versionadded:: 3.4
 
+**Source code:** :source:`Lib/asyncio/`
+
+--------------
 
 This module provides infrastructure for writing single-threaded concurrent
 code using coroutines, multiplexing I/O access over sockets and other
