Add a warning regarding nginx proxy_pass

[Wardormeur]

Hi !
In the nginx config documentation, the proxy_pass redirect to http://127.0.0.1:4567;
By setting a "/" at the end of that url, url are decoded (https://serverfault.com/questions/459369/disabling-url-decoding-in-nginx-proxy#comment610623_463932) and hence vulnerable to XSS (NodeBB/NodeBB#4192).
It's fairly easy to make the mistake of putting an ending slash (and the proxy still work), so that a warning in the doc would be welcome.
Apart from ours instances, I've spotted a couple of websites having the same issue :)

[julianlam]

Hello @Wardormeur -- very good to know! I will add the warning today, or if you'd like you can edit the files in GitHub themselves and submit a pull request 😄