Add a npm bundle size badge (minified + gzipped) to README

This lets people preview the latest version's bundle's size.

README.md

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 # Alpine.js
 
+![npm bundle size](https://img.shields.io/bundlephobia/minzip/alpinejs)
+
 Alpine.js offers you the reactive and declarative nature of big frameworks like Vue or React at a much lower cost.
 
 You get to keep your DOM, and sprinkle in behavior as you see fit.