mention num.clients for rproxy

contrib/nginx/copyparty.conf

@@ -1,3 +1,8 @@
+# when running copyparty behind a reverse-proxy,
+# make sure that copyparty allows at least as many clients as the proxy does,
+# so run copyparty with -nc 512 if your nginx has the default limits
+# (worker_processes 1, worker_connections 512)
+
 upstream cpp {
 	server 127.0.0.1:3923;
 	keepalive 120;