bpo-25008: Deprecate smtpd and point to aiosmtpd (#274)

* bpo-25008: Deprecate smtpd and point to aiosmtpd.

* Simplify the aiosmtpd URL.
diff --git a/Doc/library/smtpd.rst b/Doc/library/smtpd.rst
index e383201..85ee8a7 100644
--- a/Doc/library/smtpd.rst
+++ b/Doc/library/smtpd.rst
@@ -13,6 +13,12 @@
 
 This module offers several classes to implement SMTP (email) servers.
 
+.. seealso::
+
+    The `aiosmtpd <http://aiosmtpd.readthedocs.io/>`_ package is a recommended
+    replacement for this module.  It is based on :mod:`asyncio` and provides a
+    more straightforward API.  :mod:`smtpd` should be considered deprecated.
+
 Several server implementations are present; one is a generic
 do-nothing implementation, which can be overridden, while the other two offer
 specific mail-sending strategies.