doc: mention MTU change

README.md

@@ -49,6 +49,7 @@ But you also need to make following adjustments to wireguard client config:
 
 1. Use bind address of the dtlspipe client as endpoint for wireguard connection.
 2. Exclude dtlspipe server address from `AllowedIPs` in the wireguard client config. [This calculator](https://www.procustodibus.com/blog/2021/03/wireguard-allowedips-calculator/) may help you. Example for server address `203.0.113.11`:
+3. Use smaller MTU for wireguard tunnel, add `MTU = 1280` to the `[Peer]` section of wireguard client tunnel config.
 
 ```
 AllowedIPs = 0.0.0.0/1, 128.0.0.0/2, 192.0.0.0/5, 200.0.0.0/7, 202.0.0.0/8, 203.0.0.0/18, 203.0.64.0/19, 203.0.96.0/20, 203.0.112.0/24, 203.0.113.0/29, 203.0.113.8/31, 203.0.113.10/32, 203.0.113.12/30, 203.0.113.16/28, 203.0.113.32/27, 203.0.113.64/26, 203.0.113.128/25, 203.0.114.0/23, 203.0.116.0/22, 203.0.120.0/21, 203.0.128.0/17, 203.1.0.0/16, 203.2.0.0/15, 203.4.0.0/14, 203.8.0.0/13, 203.16.0.0/12, 203.32.0.0/11, 203.64.0.0/10, 203.128.0.0/9, 204.0.0.0/6, 208.0.0.0/4, 224.0.0.0/3, ::/0