Add copyright notice as per required by the OpenJS Foundation

Resolves json-schema-org/community#321

Author: Relequestual

README.md

@@ -96,3 +96,7 @@ Here are our top sponsors. You could be next! [[Become a sponsor](https://openco
 ## License
 
 The contents of this repository are [licensed under](./LICENSE) either the BSD 3-clause license *or* the Academic Free License v3.0.
+
+## Copyright
+
+Copyright Contributors to the JSON Schema project.