doc: minimal documentation for Emeritus status

Include a high-level explanation of how Collaborators are identified for
Emeritus status. This is intended to supply the minimum amount of
information to being assigning Emeritus status to inactive
Collaborators. The documentation may be expanded subsequently.

PR-URL: https://github.com/nodejs/node/pull/13421


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>