docs: add npm downloads rate badge to README.md file (#7127)

README.md

@@ -25,6 +25,9 @@
   <a href="https://github.com/excalidraw/excalidraw/blob/master/LICENSE">
     <img alt="Excalidraw is released under the MIT license." src="https://img.shields.io/badge/license-MIT-blue.svg"  />
   </a>
+  <a href="https://www.npmjs.com/package/@excalidraw/excalidraw">
+    <img alt="npm downloads/month" src="https://img.shields.io/npm/dm/@excalidraw/excalidraw"  />
+  </a>
   <a href="https://docs.excalidraw.com/docs/introduction/contributing">
     <img alt="PRs welcome!" src="https://img.shields.io/badge/PRs-welcome-brightgreen.svg?style=flat"  />
   </a>