Add UPGRADE-3.0.md file with descriptions of new features / changes

UPGRADE-3.0.md

@@ -0,0 +1,32 @@
+UPGRADE FROM 2.1 to 3.0
+=======================
+
+### Serialization
+
+  * you can now define a Serializer service and callback for indexing. All providers and listeners will use it.
+
+    ```yml
+    serializer:
+        callback_class: FOS\ElasticaBundle\Serializer\Callback
+        serializer: serializer
+    ```
+
+### Mapping
+
+  * you do not have to setup any mapping anymore if you use a Serializer, properties are no more indexed only if
+    they are mapped. So this kind of configuration became valid:
+
+    ```yml
+    serializer:
+        callback_class: FOS\ElasticaBundle\Serializer\Callback
+        serializer: serializer
+    indexes:
+        acme:
+            client: default
+            types:
+                Article:
+                    persistence:
+                        driver: orm
+                        model:  Acme\Bundle\CoreBundle\Entity\Article
+                        provider: ~
+    ```