Mention the license in the README

Author: bbatsov

README.md

@@ -59,3 +59,7 @@ At this point you can point your browser to <http://127.0.0.1:4000> and view the
 locally running site.
 
 **Note:** If you are using Ruby version 3 or above, you will need to add the `webrick` gem to the Gemfile prior to running `bundle install`.
+
+## License
+
+All content here is licensed under the [CC0 ("No Rights Reserved")](https://creativecommons.org/share-your-work/public-domain/cc0/) license.