Added changelog

Author: szhajdu

CHANGELOG-1.0.md

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+# Changelog
+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).
+
+## [1.0.0] - 2020-10-13
+### Added
+- Added Slim v3 support.