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>

CONTRIBUTING.md

@@ -6,6 +6,9 @@ When opening new issues or commenting on existing issues on this repository
 please make sure discussions are related to concrete technical issues with the
 Node.js software.
 
+For general help using Node.js, please file an issue at the
+[Node.js help repository](https://github.com/nodejs/help/issues).
+
 Discussion of non-technical topics including subjects like intellectual
 property, trademark and high level project questions should move to the
 [node-forward discussions repository](https://github.com/node-forward/discussions)