=doc #3685 Add FAQ to documentation

* The links at http://akka.io will changed to point at this faq
* Some rewording of guarantees/reliability

akka-docs/rst/additional/code/docs/faq/Faq.scala

@@ -0,0 +1,27 @@
+package docs.faq
+
+import akka.actor.Actor
+
+//#exhaustiveness-check
+object MyActor {
+  // these are the messages we accept
+  sealed abstract trait Message
+  case class FooMessage(foo: String) extends Message
+  case class BarMessage(bar: Int) extends Message
+
+  // these are the replies we send
+  sealed abstract trait Reply
+  case class BazMessage(baz: String) extends Reply
+}
+
+class MyActor extends Actor {
+  import MyActor._
+  def receive = {
+    case message: Message ⇒ message match {
+      case BarMessage(bar) => sender ! BazMessage("Got " + bar)
+      // warning here: 
+      // "match may not be exhaustive. It would fail on the following input: FooMessage(_)"
+    }
+  }
+}
+//#exhaustiveness-check