Add a changelog file (#39)

* Add a changelog file
* Add #39 to changelog

Author: stlehmann

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/).
+
+## [Unreleased]
+
+### Added
+* [#39](https://github.com/stlehmann/PyQt5-stubs/pull/39) Add this changelog file
+
+### Changed
+
+* [#38](https://github.com/stlehmann/PyQt5-stubs/pull/38) Changed license to GPLv3 to be compilient with PyQt5 license
+
+### Fixed