Add Documentation about configuration of the x-forwarded-* header

[alexander-schranz]

If somebody using the suluheadlessbundle in a setup where sulu is running on different domain as the actual website. It is required that the application requesting sulu will send also the x-forwarded-* headers and that this application is registered as trusted proxy: https://symfony.com/doc/current/deployment/proxies.html. So sulu generates the correct urls for the frontend application.