Adding example of headOption operator (#29717)

Co-authored-by: Johan Andrén <johan@markatta.com>

akka-docs/src/test/scala/docs/stream/operators/sink/HeadOption.scala

@@ -0,0 +1,23 @@
+/*
+ * Copyright (C) 2020 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package docs.stream.operators.sink
+
+import akka.actor.ActorSystem
+import akka.stream.scaladsl.{ Sink, Source }
+
+import scala.concurrent.{ ExecutionContextExecutor, Future }
+
+object HeadOption {
+  implicit val system: ActorSystem = ???
+  implicit val ec: ExecutionContextExecutor = system.dispatcher
+  def headOptionExample(): Unit = {
+    //#headoption
+    val source = Source.empty
+    val result: Future[Option[Int]] = source.runWith(Sink.headOption)
+    result.foreach(println)
+    //None
+    //#headoption
+  }
+}