Update website to include Build.scala example

[ghost]

It would be great to add a build.scala example in addition to build.sbt.

lazy val settings =
Defaults.coreDefaultSettings ++ PB.protobufSettings ++ Seq(
PB.runProtoc in PB.protobufConfig := (args => com.github.os72.protocjar.Protoc.runProtoc(args.toArray))
)