Groovy support/documentation

I've tried to apply this plugin to a project with a plugin written in groovy, configuring `classesDirs` to point to the groovy output, but it still did not detect any issues (it should have at least found `allProjects`). It would be awesome to have full and automatic support for groovy, but until then, I'd settle for a mention of what should actually work and what isn't supported yet, so others can find out more easily.