Draft at changelog

Still working on the release notes, but here's the changelog for review and comment.

@opscode/server-team 
Is this for end-users or end-users+developers.  From the perspective of end-users, nothing has changed about the DRBD based HA setup.  However, the DRBD configuration is now built on top of a plugin system.  This plugin system allows the Chef HA addon package to provide alternate HA providers, such as the AWS provider.