doc: add information about CTC quorum rules

CTC quorum rules were not in writing. There was an informal
understanding between CTC members. Document the rules to avoid
differences in interpretation.

PR-URL: https://github.com/nodejs/node/pull/7813
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Julien Gilli <jgilli@nodejs.org>

GOVERNANCE.md

@@ -119,6 +119,8 @@ When an agenda item has appeared to reach a consensus, the moderator will ask
 "Does anyone object?" as a final call for dissent from the consensus.
 
 If an agenda item cannot reach a consensus, a CTC member can call for either a
-closing vote or a vote to table the issue to the next meeting. The call for a
-vote must be approved by a simple majority of the CTC or else the discussion
-will continue.
+closing vote or a vote to table the issue to the next meeting. All votes
+(including votes to close or table) pass if and only if more than 50% of the CTC
+members (excluding individuals who explicitly abstain) vote in favor. For
+example, if there are 20 CTC members, and 5 of those members indicate that they
+abstain, then 8 votes in favor are required for a resolution to pass.