Add changelog

Author: aslakhellesoy

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]
+### Added
+- Make a library
+
+[Unreleased]: https://github.com/cucumber/cucumber-json-converter/compare/c7c6e23894d5becc0c99e74564489c806c71cfa8...HEAD
+