Add note to asyncore/asynchat recommending asyncio for new code.

Doc/library/asynchat.rst

@@ -10,6 +10,9 @@
 
 --------------
 
+Note: This module exists for backwards compatibility only.  For new code we
+recommend using :module:`asyncio`.
+
 This module builds on the :mod:`asyncore` infrastructure, simplifying
 asynchronous clients and servers and making it easier to handle protocols
 whose elements are terminated by arbitrary strings, or are of variable length.

Doc/library/asyncore.rst

@@ -13,6 +13,9 @@
 
 --------------
 
+Note: This module exists for backwards compatibility only.  For new code we
+recommend using :module:`asyncio`.
+
 This module provides the basic infrastructure for writing asynchronous  socket
 service clients and servers.