ENH: adding CHANGELOG

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).
+
+| SubSection | Usage                                  |
+| :--------- | :------------------------------------- |
+| Added      | for new features.                      |
+| Changed    | for changes in existing functionality. |
+| Deprecated | for soon-to-be removed features.       |
+| Removed    | for now removed features.              |
+| Fixed      | for any bug fixes.                     |
+| Security   | in case of vulnerabilities.            |
+
+
+## [Unreleased]
+
+### Added
+- started changelog
+