feature #6021 mention routing from the database (dbu)

This PR was merged into the 2.3 branch.

Discussion
----------

mention routing from the database

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.3+
| Fixed tickets | #2186, relates to #5709

if we merge #6020, we might want to put this as subsection of the extending_routing section

Commits
-------

c458431 mention routing from the database

cookbook/map.rst.inc

@@ -150,6 +150,7 @@
   * :doc:`/cookbook/routing/custom_route_loader`
   * :doc:`/cookbook/routing/redirect_trailing_slash`
   * :doc:`/cookbook/routing/extra_information`
+  * :doc:`/cookbook/routing/routing_from_database`
 
 * :doc:`Security Authentication (Identifying/Logging in the User) </cookbook/security/index>`
 

cookbook/routing/index.rst

@@ -12,3 +12,4 @@ Routing
     custom_route_loader
     redirect_trailing_slash
     extra_information
+    routing_from_database

cookbook/routing/routing_from_database.rst

@@ -0,0 +1,41 @@
+.. index::
+   single: Routing; Extra Information
+
+Looking up Routes from a Database: Symfony CMF DynamicRouter
+============================================================
+
+The core Symfony Routing System is excellent at handling complex sets
+of routes. A highly optimized routing cache is dumped during
+deployments.
+
+However, when working with large amounts of data that each need a nice
+readable URL (e.g. for search engine optimization purposes), the routing
+can get slowed down. Additionally, if routes need to be edited by users,
+the route cache would need to be rebuilt frequently.
+
+For these cases, the ``DynamicRouter`` offers an alternative approach:
+
+* Routes are stored in a database;
+* There is a database index on the path field, the lookup scales to huge
+  numbers of different routes;
+* Writes only affect the index of the database, which is very efficient.
+
+When all routes are known during deploy time and the number is not too
+high, using a :doc:`custom route loader <custom_route_loader>` is the
+preferred way to add more routes. When working with just one type of
+objects, a slug parameter on the object and the ``@ParamConverter``
+annotation work fine (see FrameworkExtraBundle_) .
+
+The ``DynamicRouter`` is useful when you need ``Route`` objects with
+the full feature set of Symfony. Each route can define a specific
+controller so you can decouple the URL structure from your application
+logic.
+
+The DynamicRouter comes with built-in support for Doctrine ORM and Doctrine
+PHPCR-ODM but offers the ``ContentRepositoryInterface`` to write a custom
+loader, e.g. for another database type or a REST API or anything else.
+
+The DynamicRouter is explained in the `Symfony CMF documentation`_.
+
+.. _FrameworkExtraBundle: https://symfony.com/doc/current/bundles/SensioFrameworkExtraBundle/annotations/converters.html
+.. _`Symfony CMF documentation`: http://symfony.com/doc/master/cmf/book/routing.html