Add codecov badge

README.md

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 [中文文档](./README_zh.md)
 
 - [![Build Status](https://travis-ci.org/aliyun/aliyun-openapi-cpp-sdk.svg?branch=master)](https://travis-ci.org/aliyun/aliyun-openapi-cpp-sdk)
+- [![codecov](https://codecov.io/gh/aliyun/aliyun-openapi-cpp-sdk/branch/master/graph/badge.svg)](https://codecov.io/gh/aliyun/aliyun-openapi-cpp-sdk)
 
 The Alibaba Cloud C++ Software Development Kit (SDK) allows you to access Alibaba Cloud services such as Elastic Compute Service (ECS), Server Load Balancer (SLB), and CloudMonitor. You can access Alibaba Cloud services without the need to handle API related tasks, such as signing and constructing your requests.
 

README_zh.md

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 # 阿里云开发者C++工具套件
 
 - [![Build Status](https://travis-ci.org/aliyun/aliyun-openapi-cpp-sdk.svg?branch=master)](https://travis-ci.org/aliyun/aliyun-openapi-cpp-sdk)
+- [![codecov](https://codecov.io/gh/aliyun/aliyun-openapi-cpp-sdk/branch/master/graph/badge.svg)](https://codecov.io/gh/aliyun/aliyun-openapi-cpp-sdk)
 
 欢迎使用阿里云开发者工具套件（SDK）。