Update README

README.md

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 [![](https://circleci.com/gh/ossrs/srs/tree/4.0release.svg?style=svg&circle-token=1ef1d5b5b0cde6c8c282ed856a18199f9e8f85a9)](https://circleci.com/gh/ossrs/srs/tree/4.0release)
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 SRS/4.0，[Leo][release4]，是一个简单高效的实时视频服务器，支持RTMP/WebRTC/HLS/HTTP-FLV/SRT。