doc: link onboarding to contributing guide

Replace the description of the commit message requirements in
onboarding.md with a link to the commit message requirements as they
appear in the CONTRIBUTING.md.

Advantages include:

* Only one place to keep the commit message requirements up to date
* Most collaborators being onboarded will already have several commits
  in their name and already be familiar with the requirements. So
  repeating information here makes finding the new information (about
  metadata, for example) harder to find.

PR-URL: https://github.com/nodejs/node/pull/8529


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: James M Snell <jasnell@gmail.com>