Update firststeps-rp.md

docs/firststeps-rp.md

@@ -15,6 +15,11 @@ This will also change the bindings inside the Nginx container! This is important
 
 Recreate affected containers by running `docker-compose up -d`.
 
+**Important information, please read them carefully!**
+
+!!! info
+    If you plan to use a reverse proxy and want to use another server name that is **not** MAILCOW_HOSTNAME, you need to read **Adding additional server names for mailcow UI** at the bottom of this page.
+
 !!! warning
     Make sure you run `generate_config.sh` before you enable any site configuration examples below.
     The script `generate_config.sh` copies snake-oil certificates to the correct location, so the services will not fail to start due to missing files.