meta: clarify collaborators & ctc members relationships

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

meta

Description of change

Clarifying collaborators & ctc members relationships inspired by this conversation https://github.com/nodejs/node/pull/7183#discussion_r73786541

This patches only the README, so I removed test relevant checks :-)

R= @jasnell