Document the way to support projects deploying without the composer.json

[stof]

See symfony/symfony#23950

In Symfony 3.3+, the kernel relies on detecting the composer.json file to detect the project dir.
If you delete the composer.json file from your deployed artifact, this logic will break. The solution in this case is to overwrite getProjectDir in your kernel (similar to the way customizing the cache dir is done btw).
this should be documented.