Add unreleased section

Author: pluma

CHANGELOG.md

@@ -5,6 +5,8 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
 ## [6.8.0] - 2018-11-07
 
 ### Changed
@@ -387,6 +389,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
   Graph methods now only return the relevant part of the response body.
 
+[unreleased]: https://github.com/arangodb/arangojs/compare/v6.8.0...HEAD
 [6.8.0]: https://github.com/arangodb/arangojs/compare/v6.7.0...v6.8.0
 [6.7.0]: https://github.com/arangodb/arangojs/compare/v6.6.0...v6.7.0
 [6.6.0]: https://github.com/arangodb/arangojs/compare/v6.5.1...v6.6.0