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: https://github.com/nodejs/node/pull/12148
Ref: https://github.com/nodejs/node/issues/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>