The ReadMe file gives you an introduction to the product and quick links to other relevant documentation. It also includes quick start guides to get you up and running with the command line tool as soon as possible.
The API documentation is a detailed programmer's guide to the Objects and methods available via the Java API.
The release notes give a version by version breakdown of new features, updates and bug fixes. They will also notify you of any unavoidable API changes that may require you to update your own code.