doc: replace node-forward link in CONTRIBUTING.md

Direct high-level discussion and questions about IP and trademark policy
to the TSC repository.

PR-URL: https://github.com/nodejs/node/pull/5227


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Rod Vagg <r@va.gg>