move code to src/test

* so that it compiles and tests pass
* fix some additional snip references in getting started

akka-docs/src/test/java/jdocs/actor/ImmutableMessage.java

@@ -0,0 +1,28 @@
+/**
+ * Copyright (C) 2009-2017 Lightbend Inc. <http://www.lightbend.com>
+ */
+package jdocs.actor;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+//#immutable-message
+public class ImmutableMessage {
+  private final int sequenceNumber;
+  private final List<String> values;
+
+  public ImmutableMessage(int sequenceNumber, List<String> values) {
+    this.sequenceNumber = sequenceNumber;
+    this.values = Collections.unmodifiableList(new ArrayList<String>(values));
+  }
+
+  public int getSequenceNumber() {
+    return sequenceNumber;
+  }
+
+  public List<String> getValues() {
+    return values;
+  }
+}
+//#immutable-message