docs: add badges

Author: YuskaWu

README.md

@@ -1,5 +1,9 @@
 # github-corner-element
 
+[![Published on webcomponents.org](https://img.shields.io/badge/webcomponents.org-published-blue.svg)](https://www.webcomponents.org/element/github-corner-element)
+[![npm](https://img.shields.io/npm/v/github-corner-element)](https://www.npmjs.com/package/github-corner-element)
+![npm bundle size](https://img.shields.io/bundlephobia/minzip/github-corner-element)
+
 A web component for the corner banner of GitHub, inspired by [Tim Holman](https://github.com/tholman)'s [GitHub Corners](https://github.com/tholman/github-corners).
 
 [Demo](https://yuskawu.github.io/github-corner-element/example)