There is no such document until now. But it is not so difficult. Check out the following modules from cvs and import them as projects in your Eclipse environment :
If you run the build.xml file of the org.jbpm.core project this will checkout and install a jbpm installation in the plugin (check the properties in the build file for a particular jbpm release).
Then you run an Eclipse workbench and you are started...
Have fun and don't hesitate to give feedback and/or bugreports,