Added the missing versionadded directive

Author: javiereguiluz

routing/debug.rst

@@ -33,11 +33,13 @@ the route name as the command argument:
 
     # or use part of the name to search for routes
     $ php bin/console debug:router blo
-    
+
       Select one of the matching routes:
       [0] blog
       [1] blog_show
-     >
+
+.. versionadded:: 4.1
+    The feature to look for partial route names was introduced in Symfony 4.1.
 
 Likewise, if you want to test whether a URL matches a given route, use the
 ``router:match`` command. This is useful to debug routing issues and find out