The RDBMS support has been added to the main Extension Library project in the most recent release, v901_07. See the release notes and/or the readme file for more details,
[ExtLib_901_release7][1] .
This project only contains sample materials to demonstrate using that RDBMS support in XPages. Those sample materials are the XPagesJDBC.nsf demonstration application and an OSGi plugin wrapping Apache Derby.
Check out the project documentation for more details.
[1] http://openntf.org/main.nsf/project.xsp?r=project/XPages%20Extension%20Library/releases/6A9595199D65774D86257D04004DF08E