Add a configuration example for build.scala

Hi.

I have written a configuration example for `build.scala` users. This example should be helpful especially for developers using sbt multi project.

Then, I have added descriptions about the meaning of `scalateOverwrite` setting and the default value of `TemplateConfig.packagePrefix` for advanced users.

The new sample code described in `README.md` is tested under `sbt 0.13.2`.

Best regards.