Document npm download badges

Add weekly and total npm download badges near the README install command so the package metrics are visible from GitHub and npm without opening the package page.

Author: LEON-gittech

README.md

@@ -12,6 +12,11 @@
   Install with <code>npm install -g @leonw24/open-codex</code>, then run <code>open-codex</code>.
 </p>
 
+<p align="center">
+  <a href="https://www.npmjs.com/package/@leonw24/open-codex"><img alt="npm weekly downloads" src="https://img.shields.io/npm/dw/%40leonw24%2Fopen-codex?label=weekly%20downloads" /></a>
+  <a href="https://www.npmjs.com/package/@leonw24/open-codex"><img alt="npm total downloads" src="https://img.shields.io/npm/dt/%40leonw24%2Fopen-codex?label=total%20downloads" /></a>
+</p>
+
 <p align="center">
   <a href="#english"><img alt="English" src="https://img.shields.io/badge/English-default-111111?style=for-the-badge" /></a>
   <a href="#简体中文"><img alt="简体中文" src="https://img.shields.io/badge/简体中文-switch-444444?style=for-the-badge" /></a>