Update TURN.md with infos about using username / password instead of the static turnserver secret

TURN.md

@@ -26,6 +26,12 @@ turn_secret = ""
 
 # the default
 turn_ttl = 86400 # 1 Day
+
+# If you have your TURN server configured to use a username and password
+# you can provide these information too. In this case comment out
+# turn_secret above!
+#turn_username = ""
+#turn_password = ""
 ```
 
 ## Apply setting