Update changelog according to newly released 2.0.0

Author: Konstantin Nazarov

CHANGELOG.md

@@ -4,7 +4,7 @@ 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]
+## [2.0.0] - 2019-10-04
 ### Added
 - Major rewrite since version 1.x
 - Ability to be used with internal http server and an nginx upstream module