Added the missing versionadded directive

Author: javiereguiluz

serializer.rst

@@ -136,6 +136,9 @@ to your class and choose which groups to use when serializing::
 .. tip::
 
     The value of the ``groups`` key can be a single string, or an array of strings.
+    
+    .. versionadded:: 4.2
+        The option to pass a single string to ``groups`` was introduced in Symfony 4.2.
 
 In addition to the ``@Groups`` annotation, the Serializer component also
 supports YAML or XML files. These files are automatically loaded when being