Update README.md

Author: lewzylu

README.md

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 # COS-PHP-SDK-V5
 腾讯云COS-PHP-SDK-V5（[XML API](https://cloud.tencent.com/document/product/436/7751)）
 
+[![Latest Stable Version](https://poser.pugx.org/qcloud/cos-sdk-v5/v/stable)](https://packagist.org/packages/qcloud/cos-sdk-v5)
 [![Total Downloads](https://img.shields.io/packagist/dt/qcloud/cos-sdk-v5.svg?style=flat)](https://packagist.org/packages/qcloud/cos-sdk-v5)
 [![Build Status](https://travis-ci.org/tencentyun/cos-php-sdk-v5.svg?branch=master)](https://travis-ci.org/tencentyun/cos-php-sdk-v5)