Document that the ?include parameter is needed for this to work

I think this was addressed by the changes to the README and project page in 📌 Document what this module actually does and what the settings mean Fixed . The `?include` parameter is part of JSON:API and not specific to this module.

That said, it is now shown clearly in the example URL on the project page and in the README.

It's also worth noting (and now added to the README) that you can use JSON:API Extras module to set the include parameter automatically so you don't need it in the URL.