[Routing] added missing "version-added" for i18n routes

Author: HeahDude

routing.rst

@@ -38,7 +38,7 @@ following configuration file:
     controllers:
         resource: '../../src/Controller/'
         type: annotation
-        
+
     kernel:
         resource: ../../src/Kernel.php
         type: annotation
@@ -1655,6 +1655,10 @@ these routes.
 Localized Routes (i18n)
 -----------------------
 
+..version-added:: 4.1
+
+    The i18n routing has been introduced in Symfony 4.1.
+
 If your application is translated into multiple languages, each route can define
 a different URL per each :doc:`translation locale </translation/locale>`. This
 avoids the need for duplicating routes, which also reduces the potential bugs: