Add external reverse proxy ACME challenge passthrough for Caddy

docs/post_installation/reverse-proxy/r_p-caddy2.en.md

@@ -66,3 +66,31 @@ This script could be called as a cronjob every hour:
 ```bash
 0 * * * * /bin/bash /path/to/script/deploy-certs.sh  >/dev/null 2>&1
 ```
+
+Alternatively, if you are using Caddy as an external reverse proxy and do not wish for it to manage the certs for Mailcow, you may include some lines to pass through the ACME challenges to acme-mailcow to allow it to manage certificates. Note that this configuration is only to allow the ACME client to fetch SSL certificates for other functions such as IMAP, etc. Caddy will still be performing the actual SSL/TLS termination, SSL certificate management and traffic forwarding for HTTP/HTTPS.
+
+``` hl_lines="1 3 13"
+
+MAILCOW_HOSTNAME autodiscover.MAILCOW_HOSTNAME autoconfig.MAILCOW_HOSTNAME {
+        log {
+                output file /var/log/caddy/MAILCOW_HOSTNAME.log {
+                        roll_disabled
+                        roll_size 512M
+                        roll_uncompressed
+                        roll_local_time
+                        roll_keep 3
+                        roll_keep_for 48h
+                }
+        }
+
+        handle /.well-known/acme-challenge* {
+            root * /acme
+        }
+
+        handle /var/www/acme* {
+            root * /var/www
+        }
+
+        reverse_proxy 127.0.0.1:HTTP_BIND
+}
+```