📝 Add changelog

changelog.md

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+# Changelog of the Dynamic Badges Action
+
+## [Dynamic Badges Action 1.2.0](https://github.com/Schneegans/dynamic-badges-action/tree/v1.2.0)
+
+**Release Date:** 2022-03-26
+
+#### Changes
+
+* The action does not log the response of writing the Gist anymore.
+* Added this changelog.
+
+
+## [Dynamic Badges Action 1.1.0](https://github.com/Schneegans/dynamic-badges-action/tree/v1.1.0)
+
+**Release Date:** 2021-06-16
+
+#### Changes
+
+* The action now logs an error message when writing the Gist failed.
+* Used new API for setting environment variables in the README.md examples.
+
+
+## [Dynamic Badges Action 1.0.0](https://github.com/Schneegans/dynamic-badges-action/tree/v1.0.0)
+
+**Release Date:** 2020-08-16
+
+#### Changes
+
+* Initial publication on GitHub.
+