[Routing] Issue #12637 Added deprecation message

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #12637
| License       | MIT
| Doc PR        | n/a
Any special reason? I am just following the Symfony Standard Conventions and the Universal Clean Code Guide (code as thin as possible)...

http://symfony.com/doc/current/contributing/code/conventions.html#deprecations