add to your build.sbt:
//compiles your CoffeeScript files to resource_managed/main/webapp/js/
(resourceManaged in (Compile, CoffeeKeys.coffee)) <<= (resourceManaged in Compile)(_ / "webapp" / "js")
//makes ALL files in resource_managed/main/webapp as static file available
com.github.siasia.PluginKeys.webappResources in Compile <+= (resourceManaged in Compile)(_ / "webapp" )
src/main/coffee/example.coffee will be available at http://localhost:8080/js/example.js