Document the option to relativize source maps

[sjrd]

See scala-js/scala-js#993

[fdietze]

Probably useful snippet to include in the docs:
build.sbt:

scalacOptions += {
  val local = baseDirectory.value.toURI
  val remote = s"https://raw.githubusercontent.com/<organization>/<repo>/${git.gitHeadCommit.value.get}/"
  s"-P:scalajs:mapSourceURI:$local->$remote"
}

plugins.sbt:

addSbtPlugin("com.typesafe.sbt" % "sbt-git" % "0.9.3")