create changelog (#184)

Author: Levi Jamt

CHANGELOG.md

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+# Change log
+All notable changes to Cosim Demo App will be documented in this file. This includes new features, bug fixes and breaking changes. 
+For a more detailed list of all changes, click the header links for each version. 
+
+This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html)
+
+### v0.7.0 – 2020-06-08
+First public release.
+
+