We've seen quite a few instances of users using the old `play.Project` constructor *and* using `playJavaSettings/playScalaSettings`.  It typically happens in multi module projects where these settings are added as base settings to common settings lists.  I think we need to be really explicit here, make a point of saying that to switch to `addPlugins(PlayScala)`, you must ensure that you remove `playScalaSettings` from your settings, as the two are not compatible.