Update README to indicate port 80 caeveat.

[ollien]

I found that if I had

server {
      listen 80;
      server_name my_website.com;
      return 301 https://$server_name$request_uri;
}

.well-known/acme-certs would 404, as it would attempt to redirect to my domain and fail, since the server_name block is more specific.

The README now documents this caveat, since it is already handled by the config.

I would open an issue, but they are disabled, so I figured this is the next best thing. Sorry if this is a bit out of form.

[staticfloat]

Thanks for this @ollien! Not out of form at all, I'm actually not sure why issues are disabled; I've enabled them again.