Bump copyright in README

Author: mpdude

README.md

@@ -155,4 +155,4 @@ developer looking for new challenges, we'd like to hear from you!
 - <https://www.webfactory.de>
 - <https://twitter.com/webfactory>
 
-Copyright 2019 – 2020 webfactory GmbH, Bonn. Code released under [the MIT license](LICENSE).
+Copyright 2019 – 2021 webfactory GmbH, Bonn. Code released under [the MIT license](LICENSE).