Improve yaml documentacion Fix #100

Extrtacted from https://github.com/schmittjoh/serializer/blob/master/tests/JMS/Serializer/Tests/Metadata/Driver/yml/ObjectWithVirtualProperties.yml

doc/reference/yml_reference.rst

@@ -15,7 +15,9 @@ YAML Reference
             map:
                 some-value: ClassName
         virtual_properties:
-            getSomeProperty: {}
+            getSomeProperty:
+                serialized_name: foo
+                type: integer
         xml_namespaces:
             "": http://your.default.namespace
             atom: http://www.w3.org/2005/Atom