DevTools: update the README.md to include the npm version.

Change-Id: Ibb20b53b9e54e7612e7fe8b9c4c385eeb0f95741
Reviewed-on: https://chromium-review.googlesource.com/c/1357547Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612843}

third_party/blink/renderer/devtools/readme.md

 # Chrome DevTools frontend
 
+<!-- [START badges] -->
+[![NPM package](https://img.shields.io/npm/v/chrome-devtools-frontend.svg)](https://npmjs.org/package/chrome-devtools-frontend)
+<!-- [END badges] -->
+
 The client-side of the Chrome DevTools, including all JS & CSS to run the DevTools webapp.
 
 It is available on NPM as the [chrome-devtools-frontend](https://www.npmjs.com/package/chrome-devtools-frontend) package. It's not currently available via CJS or ES2015 modules, so consuming this package in other tools may require [some effort](https://github.com/paulirish/devtools-timeline-model/blob/master/index.js).