Add and update CHANGELOG

Author: mcduffie

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).
+
+## [0.2.0] - 2022-07-13
+
+### Added 
+
+- [#1](https://github.com/unity-sds/unity-example-application/issues/1) - Rewrite example notebook to use updated stage_in/stage_out mechanism of unity-app-generator.
+- [#2](https://github.com/unity-sds/unity-example-application/issues/2) - Use unity-py in example notebook for input and output of STAC files for stage-in and stage-out
+- [#3](https://github.com/unity-sds/unity-example-application/issues/3) - Update unity-example-notebook README for release