Not just a link to the Javadocs; preferably a "getting started" page that covers the most common use cases with plenty of plug and play examples (if such a thing is even possible in your case).
Also, getting back to the API reference, I'm grateful when there is a roadmap or overview of the class/package structure so I don't have to crawl through the entire thing.
I wish more java projects had python-style documentation. I can probably figure out how to do things eventually by wading through Javadocs, but they can be really frustrating if all I need to know is how to accomplish the core activity of the project.
For rest-api https://github.com/apidoc/apidoc
Within the doc you can add examples for usage and response.
But a some common pages with Tutorials / HowTos are nice and should be added to a good documentation. (it depend on the complexity of your project and your audience).