Added nginx option to server requirements

[steveroot]

I almost skipped YOURLS as an option as it only said Apache, but then I spotted a suggested config for Nginx. I haven't tested that, so this is just a link to that config so other new readers can read further

[LeoColomb]

Thanks a lot for your pull request @steveroot.
That being said, I'd prefer to avoid mentioning Nginx here:

• The main reason for keeping Apache httpd only is that YOURLS comes with an automatic setup for Apache server.
• Nginx is not a requirement per se, and this list is only about requirements. Sure we are talking about http servers, but as soon as one is mentioned the other is unneeded.
• And finally, the users who are skilled enough to manage Nginx configuration (tricker than Apache httpd), sure know that Nginx is the typical alternative.

I hope that makes sense 🙂