Add version comparison link

Author: LSchueler

CHANGELOG.md

@@ -457,7 +457,8 @@ See: [#197](https://github.com/GeoStat-Framework/GSTools/issues/197)
 First release of GSTools.
 
 
-[Unreleased]: https://github.com/GeoStat-Framework/gstools/compare/v1.5.2...HEAD
+[Unreleased]: https://github.com/GeoStat-Framework/gstools/compare/v1.6.0...HEAD
+[1.6.0]: https://github.com/GeoStat-Framework/gstools/compare/v1.5.2...v1.6.0
 [1.5.2]: https://github.com/GeoStat-Framework/gstools/compare/v1.5.1...v1.5.2
 [1.5.1]: https://github.com/GeoStat-Framework/gstools/compare/v1.5.0...v1.5.1
 [1.5.0]: https://github.com/GeoStat-Framework/gstools/compare/v1.4.1...v1.5.0