[Doc] Trust all remote proxies

[Nyholm]

Using Lift and a custom domain, you need to trust all proxies in order to make it work properly.

This is because Symfony will never see Cloudfront URL because we are using the end client IP here: https://github.com/brefphp/bref/blob/master/src/Event/Http/Psr7Bridge.php#L44

If we set this the 127.0.0.1 instead, it would work for Symfony with trusted_proxies: 127.0.0.1. But that will obviously not work for other users expecing this to be the end client IP.

We could specify $_SERVER['REMOTE_ADDR'] to be either the proxy or 127.0.0.1. If so, you would configure trusted_proxies: 127.0.0.1, REMOTE_ADDR. See Symfony Request

The $_SERVER['REMOTE_ADDR'] is currently undefined.

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I am not 100% this suggestion is safe, so we should probably use $_SERVER['REMOTE_ADDR'], but that also feels wrong. I would be happy to get some input.