add document about content_type => coderef
Checking in changes prior to tagging of version 0.05.

Changelog diff is:

diff --git a/Changes b/Changes
index 04d11f7..bfc6404 100644
--- a/Changes
+++ b/Changes
@@ -2,6 +2,10 @@ Revision history for Perl extension Plack::Middleware::Expires

 {{$NEXT}}

+0.05 2014-02-10T04:04:58Z
+
+        - accept code ref for content_type attribut (Thanks leedo)
+
 0.04 2013-08-23T05:55:00Z

         - requires 'HTTP::Date' and 'HTTP::Status'. no requires LWP