Update README.md

Author: wangtao0101

README.md

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 [![NPM version](https://img.shields.io/npm/v/react-virtualized-transfer.svg?style=flat)](https://www.npmjs.com/package/react-virtualized-transfer)
 [![Build Status](https://img.shields.io/travis/wangtao0101/react-virtualized-transfer.svg?style=flat)](https://travis-ci.org/wangtao0101/react-virtualized-transfer)
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+[![codecov](https://codecov.io/gh/wangtao0101/react-virtualized-transfer/branch/master/graph/badge.svg)](https://codecov.io/gh/wangtao0101/react-virtualized-transfer)
 
 Transfer component with virtualization based on [antd](https://github.com/ant-design/ant-design), and 10 times performance than antd transfer!