src/main/scala. You can exclude source files by name (
butler.scalain the example below) like:
excludeFilter in unmanagedSources := "butler.scala"
Read more on How to exclude .scala source file in project folder - sbt Google Groups, also checkout Classpaths - sbt Wiki.
To learn more about Scala programming, I recommend Programming in Scala: A Comprehensive Step-by-Step Guide, 2nd Edition.