[Contributing][Documentation] add order of translation formats

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | 
XML should be Xliff.

And I propose to change this order to Yaml, Xliff, Php. For these reasons:

* Xliff is far from 100% supported in Symfony
* Xliff and XML are unknown to most people, causing a lot of trouble debugging since the error messages require much experience with both Xliff and XML (for instance https://github.com/symfony/symfony/issues/11256 )
* Yaml is more readable and has always been the successor of PHP config files in Symfony, I don't see why we should change that here.

/cc @weaverryan @fabpot @javiereguiluz