Minimalist changelog

CHANGELOG.md

+# Change Log
+
+## [v0.2.0](https://transfer.hft-stuttgart.de/gitlab/co2ampel/ampel-firmware/-/releases/v0.2.0)
+
+* BUGFIX: Calibration was not applied correctly (Thanks Michael Käppler for bug finding & fixing!)
+* MQTT works again for ESP32 (board needs to be updated in Arduino IDE/PlaftormIO)
+
+## [v0.1.0](https://transfer.hft-stuttgart.de/gitlab/co2ampel/ampel-firmware/-/releases/v0.1.0)
+
+* More stable calibration, with 10s timestep
+* Allow commands from Serial/MQTT/Webserver
+* Many bugfixes
+
+## [v0.0.9](https://transfer.hft-stuttgart.de/gitlab/co2ampel/ampel-firmware/-/releases/v0.0.9)
+
+* First CO2 Ampel release.
+* Basic functionality