Add changelog

Author: thomaseizinger

CHANGELOG.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](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+## [1.1.0] - 2020-02-17
+
+## [1.0.0] - 2020-02-15
+
+### Added
+
+-   Initial release of this action!
+
+[Unreleased]: https://github.com/thomaseizinger/keep-a-changelog-new-release/compare/1.1.0...HEAD
+
+[1.1.0]: https://github.com/thomaseizinger/keep-a-changelog-new-release/compare/1.0.0...1.1.0
+
+[1.0.0]: https://github.com/thomaseizinger/keep-a-changelog-new-release/compare/8f254ca247120d87500da53956ae6c0c9d9fae3e...1.0.0
+