update readme, add chnvideo china mirror

 #Simple-RTMP-Server
 
-You can [Download Released SRS(下载发布版)](http://winlinvip.github.io/simple-rtmp-server).
+You can [Download Released SRS(下载发布版)](http://winlinvip.github.io/simple-rtmp-server)
+
+Or download from [chnvideo mirror(国内镜像)](http://demo.chnvideo.com:8085/srs/releases/)
 
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