You can add documentation at package level.
From Sun documentation:
Typically package-info.java contains only a package declaration, preceded immediately by the annotations on the package. While the file could technically contain the source code for one or more package-private classes, it would be very bad form.
It is recommended that package-info.java, if it is present, take the place of package.html for javadoc and other similar documentation generation systems. If this file is present, the documentation generation tool should look for the package documentation comment immediately preceding the (possibly annotated) package declaration in package-info.java. In this way, package-info.java becomes the sole repository for package level annotations and documentation. If, in future, it becomes desirable to add any other package-level information, this file should prove a convenient home for this information.