please document defaultScalaVersion

[medovina]

I'm trying cbt for the first time. So far it looks quite promising. But the first thing I wanted to do was to tell cbt which Scala version to use for my project, and that isn't obviously documented anywhere. I looked around in the cbt sources for a while, and eventually figured out that I can add this to my Build class:

  override def defaultScalaVersion = "2.12.3"

That works. This seems fundamentally important, so please add documentation about this to the cbt tutorial. Thanks!