Add docs and example for akka-stream operator from (#24933) (#25372)

* Add docs and example for akka-stream operator from (#24933)

* add seperate test class for code example of akka-stream from

* add copyright header

akka-docs/src/main/paradox/stream/operators/Source/from.md

@@ -4,10 +4,19 @@ Stream the values of an `Iterable`.
 
 @ref[Source operators](../index.md#source-operators)
 
+
+@@@div { .group-scala }
+
+## Signature
+
+@@signature [Source.scala]($akka$/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala) { #from }
+
+@@@
+
 ## Description
 
 Stream the values of an `Iterable`. Make sure the `Iterable` is immutable or at least not modified after being used
-as a source.
+as a source. Otherwise the stream may fail with `ConcurrentModificationException` or other more subtle errors may occur.
 
 @@@div { .callout }
 
@@ -17,3 +26,8 @@ as a source.
 
 @@@
 
+
+## Examples
+
+Java
+:  @@snip [from.java]($akka$/akka-docs/src/test/java/jdocs/stream/operators/SourceDocExamples.java) { #imports #source-from-example }

akka-docs/src/test/java/jdocs/stream/operators/SourceDocExamples.java

@@ -0,0 +1,35 @@
+/**
+ * Copyright (C) 2018 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package jdocs.stream.operators;
+
+//#imports
+import akka.NotUsed;
+import akka.actor.ActorSystem;
+import akka.stream.ActorMaterializer;
+import akka.stream.Materializer;
+import akka.stream.javadsl.Source;
+
+import java.util.Arrays;
+
+//#imports
+
+public class SourceDocExamples {
+
+    public static void fromExample() {
+        //#source-from-example
+        final ActorSystem system = ActorSystem.create("SourceFromExample");
+        final Materializer materializer = ActorMaterializer.create(system);
+
+        Source<Integer, NotUsed> ints = Source.from(Arrays.asList(0, 1, 2, 3, 4, 5));
+        ints.runForeach(System.out::println, materializer);
+
+        String text = "Perfection is finally attained not when there is no longer more to add," +
+                "but when there is no longer anything to take away.";
+        Source<String, NotUsed> words = Source.from(Arrays.asList(text.split("\\s")));
+        words.runForeach(System.out::println, materializer);
+        //#source-from-example
+    }
+
+}