Added the "versionadded: 2.7" directive

Author: javiereguiluz

reference/dic_tags.rst

@@ -231,6 +231,9 @@ The tagged service will be removed from the container if
 auto_alias
 ----------
 
+.. versionadded:: 2.7
+    The ``auto_alias`` tag was introduced in Symfony 2.7.
+
 **Purpose**: Define aliases based on the value of container parameters
 
 Consider the following configuration that defines three different but related