docs: add 2021 to license

Author: jdbruijn

README.md

@@ -84,7 +84,7 @@ Please refer to the [Security Policy on GitHub](https://github.com/vidavidorra/c
 
 This project is licensed under the [GPLv3 license](https://www.gnu.org/licenses/gpl.html).
 
-Copyright © 2020 Jeroen de Bruijn
+Copyright © 2020-2021 Jeroen de Bruijn
 
 <details><summary>License details.</summary>
 <p>