doc: update copyright date range

Author: ca-d

README.md

@@ -31,6 +31,6 @@ distributed under their [end user license agreement](CC-EULA.pdf).
 
 This project is licensed under the [Apache License 2.0](LICENSE).
 
-© 2020 OMICRON electronics GmbH
+© 2020-2021 OMICRON electronics GmbH
 
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