Bump copyright year

Author: horgh

README.md

@@ -57,7 +57,7 @@ This library uses [Semantic Versioning](https://semver.org/).
 
 ## Copyright and License
 
-This software is Copyright (c) 2018 - 2020 by MaxMind, Inc.
+This software is Copyright (c) 2018 - 2021 by MaxMind, Inc.
 
 This is free software, licensed under the [Apache License, Version
 2.0](LICENSE-APACHE) or the [MIT License](LICENSE-MIT), at your option.