docs: update changelog for 2.6.0

Author: ungoldman

CHANGELOG.md

@@ -5,7 +5,7 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
-## Unreleased Changes
+## [2.6.0](https://github.com/ungoldman/changelog-parser/compare/v2.5.0...v2.5.1) - 2018-12-18
 
 ### Features