doc: add logo to README

Adds a centered logo to the README to make it a little more festive. As
centering is not possible in pure Markdown, a bit of HTML is used.

PR-URL: nodejs/node#12148
Ref: nodejs/node#6920
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

README.md

@@ -1,6 +1,10 @@
-# Node.js
-
-[![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/nodejs/node?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge) [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/29/badge)](https://bestpractices.coreinfrastructure.org/projects/29)
+<p align="center">
+  <img alt="Node.js" src="https://nodejs.org/static/images/logo-light.svg" width="400"/>
+</p>
+<p align="center">
+  <a title="Gitter" href="https://gitter.im/nodejs/node?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge"><img src="https://badges.gitter.im/Join%20Chat.svg"></a>
+  <a title="CII Best Practices" href="https://bestpractices.coreinfrastructure.org/projects/29"><img src="https://bestpractices.coreinfrastructure.org/projects/29/badge"></a>
+</p>
 
 Node.js is a JavaScript runtime built on Chrome's V8 JavaScript engine. Node.js
 uses an event-driven, non-blocking I/O model that makes it lightweight and