Adds example to Source.completionStageSource (#28639)

akka-docs/src/test/scala/docs/stream/operators/source/FutureSource.scala

@@ -0,0 +1,28 @@
+/*
+ * Copyright (C) 2020 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package docs.stream.operators.source
+
+// #sourceFutureSource
+
+import akka.NotUsed
+import akka.stream.scaladsl.Source
+
+import scala.concurrent.Future
+
+object FutureSource {
+  def sourceCompletionStageSource(): Unit = {
+    val userRepository: UserRepository = ??? // an abstraction over the remote service
+    val userFutureSource = Source.futureSource(userRepository.loadUsers)
+    // ...
+  }
+
+  trait UserRepository {
+    def loadUsers: Future[Source[User, NotUsed]]
+  }
+
+  case class User()
+}
+
+// #sourceFutureSource