Please document which classes are in which Maven artifacts

[lukehutch]

Please provide an index for which Java classes are in which Maven artifacts. For example, I was trying to compile some code that used org.ros.message.MessageListener, and I assumed it would be part of org.ros.rosjava_core:rosjava, but it was actually part of org.ros.rosjava_bootstrap:message_generation. I had to do the following to figure that out, which is a bit of a pain:

for i in `find ~/.m2/repository/org/ros | grep '.*\.jar$'` ; do echo $i ; unzip -l $i | grep MessageListener ; done

[lukehutch]

The index could be in README.md, or if the classes are unlikely to change packages, you could include the relevant groupId and artifactId in the JavaDoc for each class.