Mention that Symfony only loads YAML routes by default

Author: javiereguiluz

routing.rst

@@ -147,6 +147,12 @@ the ``BlogController``:
             ;
         };
 
+.. versionadded:: 5.1
+
+    Starting from Symfony 5.1, by default Symfony only loads the routes defined
+    in YAML format. If you define routes in XML and/or PHP formats, update the
+    ``src/Kernel.php`` file to add support for the ``.xml`` and ``.php`` file extensions.
+
 .. _routing-matching-http-methods:
 
 Matching HTTP Methods