docs: add nightly releases badge

Author: brc-dd

README.md

@@ -2,6 +2,7 @@
 
 [![test](https://github.com/vuejs/vitepress/workflows/Test/badge.svg)](https://github.com/vuejs/vitepress/actions)
 [![npm](https://img.shields.io/npm/v/vitepress)](https://www.npmjs.com/package/vitepress)
+[![nightly releases](https://img.shields.io/badge/nightly-releases-orange)](https://nightly.akryum.dev/vuejs/vitepress)
 [![chat](https://img.shields.io/badge/chat-discord-blue?logo=discord)](https://chat.vuejs.org)
 
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