Fixed #3398 -- document the serial deprecation in the changelog (#3401)

* Fixed #3398 -- document the serial deprecation in the changelog

* fix

* sigh, fix
diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 1616947..cd74ff8 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -161,6 +161,9 @@
   and
   :meth:`~cryptography.hazmat.primitives.asymmetric.rsa.RSAPublicKey.verify`
   methods to RSA keys.
+* Deprecated the ``serial`` attribute on
+  :class:`~cryptography.x509.Certificate`, in favor of
+  :attr:`~cryptography.x509.Certificate.serial_number`.
 
 
 1.3.4 - 2016-06-03