minor #7488 Added a mention to ChainRouter from Symfony CMF (javiereguiluz)

This PR was merged into the 2.7 branch.

Discussion
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Added a mention to ChainRouter from Symfony CMF

This replaces #6020 to complete the mentions to Symfony CMF routers.

Commits
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63213a7 Added a mention to ChainRouter from Symfony CMF

Author: xabbuh

routing/custom_route_loader.rst

@@ -274,7 +274,15 @@ configuration file - you can call the
     be anything that would normally be supported by the routing configuration
     loader (YAML, XML, PHP, annotation, etc.).
 
+.. note::
+
+    For more advanced uses, check out the `ChainRouter`_ provided by the Symfony
+    CMF project. This router allows applications to use two or more routers
+    combined, for example to keep using the default Symfony routing system when
+    writing a custom router.
+
 .. _`FOSRestBundle`: https://github.com/FriendsOfSymfony/FOSRestBundle
 .. _`JMSI18nRoutingBundle`: https://github.com/schmittjoh/JMSI18nRoutingBundle
 .. _`KnpRadBundle`: https://github.com/KnpLabs/KnpRadBundle
 .. _`SonataAdminBundle`: https://github.com/sonata-project/SonataAdminBundle
+.. _`ChainRouter`: https://symfony.com/doc/current/cmf/components/routing/chain.html