Adding documentation how to map directories

Should we provide a doc page with `latest`, too? Like Akka or SBT?

### Links

* http://www.scala-sbt.org/release/docs/Detailed-Topics/Mapping-Files.html
* http://www.scala-sbt.org/0.13.1/docs/Detailed-Topics/Paths.html
* http://www.scala-sbt.org/0.13.1/api/index.html#sbt.PathFinder
* https://github.com/sbt/sbt/blob/0.14/util/io/src/main/scala/sbt/Path.scala
* https://github.com/sbt/sbt/blob/0.14/util/io/src/main/scala/sbt/NameFilter.scala
* https://github.com/sbt/sbt/blob/0.14/util/io/src/main/scala/sbt/IO.scala