Show use of parameters in routing.yml

[weaverryan]

New to 2.1, from the CHANGELOG:

• added support for placeholders in route defaults and requirements (replaced by the value set in the service container)

[weaverryan]

This would work as a nice "note" inside the routing chapter. We would show a very simple example, and then link to the "Service Parameters" part of the "Service Container" chapter by using:

:ref:`book-service-container-parameters`