Added Changelog

Author: norberttech

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
+
+- [#3](https://github.com/flow-php/etl/pull/3) - **Remove Webmozart/Assert dependency** - [@Wiktor6](https://github.com/Wiktor6)
+- Initial version of the ETL component by [@tomaszhanc](https://github.com/tomaszhanc) and [@norberttech](https://github.com/norberttech)