chore: add npm badge in readme (#31)

README.md

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 <p align="center"><sup><i>Programming for the charm of words, coding to draw the magnificent scenery.</i></sup></p>
 <p align="center"><sup>by Hitokoto Developers</sup></p>
 
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+[![npm](https://img.shields.io/npm/v/koishi-plugin-hitokoto?style=flat-square)](https://www.npmjs.com/package/koishi-plugin-hitokoto)
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 🌏 English | [简体中文](./README.zh-CN.md)
 
 A [koishi](https://github.com/koishijs/koishi) plugin to get a random "Hitokoto" Chinese sentence, which is from [一言网](https://hitokoto.cn).