DOC: add changelog file

Refs: #2

CHANGELOG.md

+# Changelog
+
+All significant changes to this project are documented in this file.
+
+The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
+and the project follows [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
+
+
+## Unreleased yet ---> [3.0.x](#)  - 2020-0x-xx
+
+* [...](#)
+
+### Added
+
+
+### Changed
+
+
+### Fixed
+
+
+### Deprecated
+
+
+### Security
+
+