Change Styling of CHANGELOG.md

The most logical way for a changeling file, according to a lot of sources, and most changelog files of open source projects, is:

Explanation

Unreleased:

Added

Changed

Removed

2.0.0: date release

Added

Changed

Removed

1.1.0: date release

Added

Changed

Removed

0.0.1: date release

Added

Changed

Removed

This ordering would make more sense since the most recent version comes up first. The unreleased is above everything else where everyone should include their most recent changes. If a new version comes out, the only thing that has to be added is a new header and the current Unreleased header can be changed to the current release.

Also this styling should be accepted by gitlab as official and should thus make use of the automatic merging scripts that gitlab implemented for changelogs.