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: #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>

GOVERNANCE.md

@@ -67,9 +67,9 @@ Typical activities of a Collaborator include:
 * participation in working groups
 * merging pull requests
 
-While the above are typical things done by Collaborators, there are no required
-activities to retain Collaborator status. There is currently no process by which
-inactive Collaborators are removed from the project.
+The CTC periodically reviews the Collaborator list to identify inactive
+Collaborators. Past Collaborators are typically given _Emeritus_ status. Emeriti
+may request that the CTC restore them to active status.
 
 ## CTC Membership