Added the versionadded directive

Author: javiereguiluz

service_container/alias_private.rst

@@ -151,6 +151,9 @@ Anonymous Services
 
     Anonymous services are only supported by the XML and YAML configuration formats.
 
+.. versionadded:: 3.3
+    The feature to configure anonymous services in YAML was introduced in Symfony 3.3.
+
 In some cases, you may want to prevent a service being used as a dependency of
 other services. This can be achieved by creating an anonymous service. These
 services are like regular services but they don't define an ID and they are