doc: update CONTRIBUTING.MD with link to V8 guide

Currently, two of the guides in the `/doc/guides` directory are actually
guides for working on the Nodei.js project. Of those, one is linked from
this page. This change adds a note to point people to the other.

PR-URL: https://github.com/nodejs/node/pull/10070


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>