docs: add install size badge

closes #3710

Author: styfle

Readme.md

@@ -3,6 +3,7 @@
   Fast, unopinionated, minimalist web framework for [node](http://nodejs.org).
 
   [![NPM Version][npm-version-image]][npm-url]
+  [![NPM Install Size][npm-install-size-image]][npm-install-size-url]
   [![NPM Downloads][npm-downloads-image]][npm-downloads-url]
 
 ```js
@@ -159,5 +160,7 @@ The current lead maintainer is [Douglas Christopher Wilson](https://github.com/d
 [github-actions-ci-url]: https://github.com/expressjs/express/actions/workflows/ci.yml
 [npm-downloads-image]: https://badgen.net/npm/dm/express
 [npm-downloads-url]: https://npmcharts.com/compare/express?minimal=true
+[npm-install-size-image]: https://badgen.net/packagephobia/install/express
+[npm-install-size-url]: https://packagephobia.com/result?p=express
 [npm-url]: https://npmjs.org/package/express
 [npm-version-image]: https://badgen.net/npm/v/express