This repository is being currently worked on to prepare for the upcoming 0.9 release. We're also re-engeineering the plugin development workflow, entirely based on Maven. When finished, documentation on how to create, build and test plugins will be available here. ETA is end of December 2015.
In the meantime, the legacy plugin code can be found in the '0.8.2' branch but be aware that the procedure to develop and build plugins will be changed soon.