Merge pull request esrrhs#55 from jiqing112/patch-2

Update README.md

README.md

@@ -20,6 +20,7 @@ Pingtunnel is a tool that send TCP/UDP traffic over ICMP.
 
 -   First prepare a server with a public IP, such as EC2 on AWS, assuming the domain name or public IP is www.yourserver.com
 -   Download the corresponding installation package from [releases](https://github.com/esrrhs/pingtunnel/releases), such as pingtunnel_linux64.zip, then decompress and execute with **root** privileges
+-   “-key” parameter is **int** type, only supports numbers between 0-2147483647
 
 ```
 sudo wget (link of latest release)
@@ -38,6 +39,8 @@ echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_all
 -   Download the corresponding installation package from [releases](https://github.com/esrrhs/pingtunnel/releases), such as pingtunnel_windows64.zip, and decompress it
 -   Then run with **administrator** privileges. The commands corresponding to different forwarding functions are as follows.
 -   If you see a log of ping pong, the connection is normal
+-   “-key” parameter is **int** type, only supports numbers between 0-2147483647
+
 
 #### Forward sock5