Added xml_root_namespace on YAML reference

doc/reference/yml_reference.rst

@@ -6,6 +6,7 @@ YAML Reference
     Vendor\MyBundle\Model\ClassName:
         exclusion_policy: ALL
         xml_root_name: foobar
+        xml_root_namespace: http://your.default.namespace
         exclude: true
         read_only: false
         access_type: public_method # defaults to property