Add examples in Scala documentation for Source.fromFuture #24933 (#25374)

* Add scala documentation for Source.fromFuture #24933

akka-docs/src/main/paradox/stream/operators/Source/fromFuture.md

@@ -26,3 +26,7 @@ If the future fails the stream is failed with that exception.
 
 @@@
 
+## Example
+Scala
+:  @@snip [SourceFromFuture.scala]($akka$/akka-docs/src/test/scala/docs/stream/operators/SourceOperators.scala) { #sourceFromFuture }
+

akka-docs/src/test/scala/docs/stream/operators/SourceOperators.scala

@@ -0,0 +1,28 @@
+/*
+ * Copyright (C) 2018 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package docs.stream.operators
+
+object SourceOperators {
+
+  def fromFuture = {
+    //#sourceFromFuture
+
+    import akka.actor.ActorSystem
+    import akka.stream.ActorMaterializer
+    import akka.{ Done, NotUsed }
+    import akka.stream.scaladsl._
+
+    import scala.concurrent.Future
+
+    implicit val system: ActorSystem = ActorSystem()
+    implicit val materializer: ActorMaterializer = ActorMaterializer()
+
+    val source: Source[Int, NotUsed] = Source.fromFuture(Future.successful(10))
+    val sink: Sink[Int, Future[Done]] = Sink.foreach((i: Int) ⇒ println(i))
+
+    val done: Future[Done] = source.runWith(sink) //10
+    //#sourceFromFuture
+  }
+}