Adding information about dhparam.pem cert

In `etc/nginx/snippets/ssl-params.conf`,
the `/etc/nginx/certs/dhparam.pem` seems to be appear out of nowhere.
This commit adding information when is the dhparam.pem was generated and
give example command how to generate the dhparam manually.

etc/nginx/snippets/ssl-params.conf

@@ -4,7 +4,10 @@
 ssl_session_timeout 1d;
 ssl_session_cache shared:SSL:10m;  # about 40000 sessions
 ssl_session_tickets off;
-ssl_dhparam /etc/nginx/certs/dhparam.pem;
 ssl_protocols TLSv1.2 TLSv1.3;
 ssl_ciphers ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384;
 ssl_prefer_server_ciphers off;
+# The dhparam.pem below is generated when you first execute `./kickstart.sh -I`
+# If you not using the kickstart.sh, you can generate it by yourself.
+# Example: openssl dhparam -out /etc/nginx/certs/dhparam.pem 2048
+ssl_dhparam /etc/nginx/certs/dhparam.pem;