docs: add npm version badge

Author: meteorlxy

README.md

@@ -2,6 +2,7 @@
 
 [![github check](https://github.com/vuepress/vuepress-next/workflows/check/badge.svg)](https://github.com/vuepress/vuepress-next/actions?query=workflow%3Acheck)
 [![github docs](https://github.com/vuepress/vuepress-next/workflows/docs/badge.svg)](https://github.com/vuepress/vuepress-next/actions?query=workflow%3Adocs)
+[![npm](https://badgen.net/npm/v/vuepress/next)](https://www.npmjs.com/package/vuepress)
 [![license](https://badgen.net/github/license/vuepress/vuepress-next)](https://github.com/vuepress/vuepress-next/blob/main/LICENSE)
 
 ## Status: Alpha