add doc about serializer
Checking in changes prior to tagging of version 0.03.

Changelog diff is:

diff --git a/Changes b/Changes
index cae08ff..69867d0 100644
--- a/Changes
+++ b/Changes
@@ -2,6 +2,10 @@ Revision history for Perl extension Plack-Middleware-Session-Simple

 {{$NEXT}}

+0.03 2014-10-20T14:20:03Z
+
+    - add serializer option
+
 0.02 2013-10-27T14:43:31Z

     - [incompatible change] change default session id generator.