Added date to CHANGELOG

CHANGELOG.md

 # Change Log
 
-## [v0.2.0](https://transfer.hft-stuttgart.de/gitlab/co2ampel/ampel-firmware/-/releases/v0.2.0)
+## [v0.2.0](https://transfer.hft-stuttgart.de/gitlab/co2ampel/ampel-firmware/-/releases/v0.2.0) 2021/06/06
 
 * 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)
+## [v0.1.0](https://transfer.hft-stuttgart.de/gitlab/co2ampel/ampel-firmware/-/releases/v0.1.0) 2021/05/19
 
 * 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)
+## [v0.0.9](https://transfer.hft-stuttgart.de/gitlab/co2ampel/ampel-firmware/-/releases/v0.0.9) 2020/12/15
 
 * First CO2 Ampel release.
 * Basic functionality