doc: write down some basic infos on chatmail-turn (#693)

Co-authored-by: l <[email protected]>

Author: missytake

README.md

@@ -180,6 +180,10 @@ The components of chatmail are:
 - [Iroh relay](https://www.iroh.computer/docs/concepts/relay) 
   which helps client devices to establish Peer-to-Peer connections 
 
+- [TURN](https://github.com/chatmail/chatmail-turn)
+  to enable relay users to start webRTC calls
+  even if a p2p connection can't be established
+
 - and the chatmaild services, explained in the next section:
 
 ### chatmaild
@@ -304,6 +308,8 @@ Chatmail address creation will be denied while this file is present.
 [Nginx](https://www.nginx.com/) listens on port 8443 (HTTPS-ALT) and 443 (HTTPS).
 Port 443 multiplexes HTTPS, IMAP and SMTP using ALPN to redirect connections to ports 8443, 465 or 993.
 [acmetool](https://hlandau.github.io/acmetool/) listens on port 80 (HTTP).
+[chatmail-turn](https://github.com/chatmail/chatmail-turn) listens on TCP port 3478 (STUN/TURN),
+and temporarily opens UDP ports when users request them. UDP port range is not restricted, any free port may be allocated.
 
 chatmail-core based apps will, however, discover all ports and configurations
 automatically by reading the [autoconfig XML file](https://www.ietf.org/archive/id/draft-bucksch-autoconfig-00.html) from the chatmail relay server.