doc: add help repo link to CONTRIBUTING.md

Direct help requests and technical issues that are not issues with
Node.js to the nodejs help repo issue tracker. Add link.

PR-URL: https://github.com/nodejs/node/pull/3233
Fixes: https://github.com/nodejs/node/issues/3185


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>