doc: add link on logo to README

This is to override GitHub's default behaviour that links to the
image's source file, which isn't very helpful in our case.

PR-URL: nodejs/node#12307
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

README.md

@@ -1,5 +1,7 @@
 <p align="center">
-  <img alt="Node.js" src="https://nodejs.org/static/images/logo-light.svg" width="400"/>
+  <a href="https://nodejs.org/">
+    <img alt="Node.js" src="https://nodejs.org/static/images/logo-light.svg" width="400"/>
+  </a>
 </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>