doc: add link to triage guide
This is a bit of a meta-pull request.
I'm interested in helping out more within the Node.js project on the heels of GHC Open Source Day mentoring.
In researching the landscape a bit, I determined that the triage role (PR for that forthcoming...) would be a great place to start, but it took me a while to find the triage guidance from README
GOVERNANCE
.doc/contributing/issues
This PR makes a direct reference to the triage guide, emulating the pattern of the collaborator guide in the previous section.