Add completion timeout example (#29673)

* Adding example for completionTimeout

* Adding header to file

* Modifying example

* Modifying duration

* Modifying syntax for duration

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

@@ -0,0 +1,23 @@
+/*
+ * Copyright (C) 2020 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package docs.stream.operators.sourceorflow
+
+import akka.Done
+import akka.actor.ActorSystem
+import akka.stream.scaladsl.{ Flow, Sink, Source }
+
+import scala.concurrent.duration._
+import scala.concurrent.{ ExecutionContextExecutor, Future }
+
+object CompletionTimeout {
+  implicit val system: ActorSystem = ???
+  implicit val ec: ExecutionContextExecutor = system.dispatcher
+  def completionTimeoutExample: Future[Done] = {
+    //#completionTimeout
+    val source = Source(1 to 10000).map(number => number * number)
+    source.completionTimeout(10.milliseconds).run()
+    //#completionTimeout
+  }
+}