readme: added zenodo doi badge

README.md

@@ -5,6 +5,7 @@
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+[![DOI](https://zenodo.org/badge/272405671.svg)](https://zenodo.org/badge/latestdoi/272405671)
 
 MQTT client triggering & reporting shutdown on [systemd](https://freedesktop.org/wiki/Software/systemd/)-based systems