improve fine-grained fec

doc/README.zh-cn.md

@@ -106,7 +106,7 @@ https://github.com/wangyu-/udp2raw-tunnel
 ### 命令选项
 ```
 UDPspeeder V2
-git version: 6f55b8a2fc    build date: Nov 19 2017 06:11:23
+git version: 3e248b414c    build date: Aug  5 2018 21:59:52
 repository: https://github.com/wangyu-/UDPspeeder
 
 usage:
@@ -131,6 +131,8 @@ advanced options:
                                           do not use if you dont know what it means.
     -i,--interval         <number>        scatter each fec group to a interval of <number> ms, to protect burst packet loss.
                                           default value: 0. do not use if you dont know what it means.
+    -f,--fec              x1:y1,x2:y2,..  similiar to -f/--fec above,fine-grained fec parameters,may help save bandwidth.
+                                          example: "-f 1:3,2:4,10:6,20:10". check repo for details
     --random-drop         <number>        simulate packet loss, unit: 0.01%. default value: 0.
     --disable-obscure     <number>        disable obscure, to save a bit bandwidth and cpu.
 developer options:
@@ -151,8 +153,6 @@ log and help options:
     --disable-color                       disable log color
     -h,--help                             print this help message
 
-
-
 ```
 ### 包发送选项，两端设置可以不同。 只影响本地包发送。
 ##### `-f` 选项