doc: add help repo link to CONTRIBUTING.md
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 <ad0310dc5270cefb6ee6879263270aed9b1e9982@me.com>
Reviewed-By: Michaël Zasso <dc852f7f42c586356f6be32b8780273088b3bded@gmail.com>
Reviewed-By: Rich Trott <28a7b6144c43947e3f532c033ff42c8a2ae08045@gmail.com>

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