The Readme file gives you an introduction to the XML Compare product and quick links to other relevant documentation. It also includes quick start guides to get you up and running with the GUI or the command line tool as soon as possible.
The API documentation is a detailed programmer's guide to the Objects and methods available via either the Java or .NET APIs.
This provides a high-level descripton of the XML Compare product, covering all the main features, and links to more detailed reference documentation and samples.
Features and properties are settings which can be used to configure both the standalone comparators/combiners and also the pipelined comparators.
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.