Docs: Reduce operator (#29641)

Co-authored-by: Arnout Engelen <github@bzzt.net>

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

@@ -0,0 +1,23 @@
+/*
+ * Copyright (C) 2009-2020 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 Reduce {
+  implicit val system: ActorSystem = ???
+  implicit val ec: ExecutionContextExecutor = system.dispatcher
+  def reduceExample: Future[Unit] = {
+    //#reduceExample
+    val source = Source(1 to 100).reduce((acc, element) => acc + element)
+    val result: Future[Int] = source.runWith(Sink.head)
+    result.map(println)
+    //5050
+    //#reduceExample
+  }
+}