doc: add link to help repo in README

Include link to Node.js help repo (for end user questions on how to use
Node.js) in the README.

PR-URL: https://github.com/nodejs/node/pull/8570


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>