Add Build.scala extra imports explanation, v2

To use the includeFilter / exludeFilter examples in Build.scala, the Assets and LessKeys symbols need to be imported.

(re-do of https://github.com/sbt/sbt-less/pull/39 to get it into a single commit)
Can we say something along the lines of "when in the context of using Play"... remember that sbt-less can be used outside of Play.