Example for mapConcat (#28243)

* Example for mapConcat

akka-docs/src/test/scala/docs/stream/operators/sourceorflow/MapConcat.scala

@@ -0,0 +1,33 @@
+/*
+ * Copyright (C) 2019 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package docs.stream.operators.sourceorflow
+
+import akka.stream.scaladsl.Source
+
+import scala.concurrent.ExecutionContext
+
+object MapConcat {
+
+  def mapConcat(): Unit = {
+    import akka.actor.ActorSystem
+
+    implicit val system: ActorSystem = ActorSystem()
+    implicit val ec: ExecutionContext = system.dispatcher
+
+    //#map-concat
+    def duplicate(i: Int): List[Int] = List(i, i)
+
+    Source(1 to 3).mapConcat(i => duplicate(i)).runForeach(println)
+    // prints:
+    // 1
+    // 1
+    // 2
+    // 2
+    // 3
+    //#map-concat
+
+  }
+
+}