Update CHANGELOG.md for Release v1.8.0

Author: Art4

CHANGELOG.md

@@ -5,7 +5,9 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## [Unreleased](https://github.com/kbsali/php-redmine-api/compare/v1.7.0...master)
+## [Unreleased](https://github.com/kbsali/php-redmine-api/compare/v1.8.0...master)
+
+## [v1.8.0](https://github.com/kbsali/php-redmine-api/compare/v1.7.0...v1.8.0) - 2021-04-19
 
 ### Added