Add initial changelog

Author: marijnvanwezel

CHANGELOG.md

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+# Changelog
+
+All notable changes to the php-cypher-dsl project will be documented in this file. A changelog has been kept from version 5.0.0 onwards.
+
+The format is based on [Keep a Changelog], and this project adheres to
+[Semantic Versioning].
+
+## 5.0.0 - To be determined
+
+### Added
+
+- Add a blank changelog.
+
+[keep a changelog]: https://keepachangelog.com/en/1.0.0/
+[semantic versioning]: https://semver.org/spec/v2.0.0.html