Shinken is not just a fork of Nagios. It has been rewritten completely as a modern distributed application while maintaining compatibility with the Nagios configuration, LiveStatus API and check plugins.
The major differences are listed on the page http://www.shinken-monitoring.org/what-is-in-shinken-not-in-nagios-and-vice-versa/.
The Changelog file is included in the source root directory of the source code distribution.