Docs: Add non-attending TSC member info (#11411)

Author: nzakas

docs/maintainer-guide/governance.md

@@ -108,6 +108,8 @@ There is no specific set of requirements or qualifications for TSC members beyon
 
 A Reviewer is invited to become a TSC member by existing TSC members. A nomination will result in discussion and then a decision by the TSC.
 
+**Non-attending TSC Members:** TSC members who will be unavailable for meetings for any period of time may choose to be considered non-attending members. Non-attending members retain their TSC status but do not count as absent from meetings or votes. For instance, if the TSC has seven members with one non-attending member, the effective TSC size is six for meetings and votes. Non-attending members may still attending TSC meetings and participate in votes, in which case they are counted as an attending TSC member.
+
 #### Process for Adding TSC Members
 
 1. Add the GitHub user to the "ESLint TSC" GitHub team
@@ -120,6 +122,11 @@ A Reviewer is invited to become a TSC member by existing TSC members. A nominati
 1. Add the TSC member to the ESLint TSC mailing list as an "Owner"
 1. Tweet congratulations to the new TSC member from the ESLint Twitter account
 
+#### Process for Adding Non-attending TSC Members
+
+1. The TSC member informs the rest of the TSC about their non-attending status
+1. The TSC member is added to the "Non-attending TSC" GitHub team
+
 #### TSC Meetings
 
 The TSC meets every other week in the [TSC Meeting](https://gitter.im/eslint/tsc-meetings) chatroom. The meeting is run by
@@ -144,7 +151,7 @@ cannot veto or remove items.
 
 No binding votes on TSC agenda items can take place without a quorum of
 TSC members present in the meeting. Quorum is achieved when more than
-half of the TSC members are present.
+half of the TSC members (minus non-attending members) are present.
 
 The TSC may invite persons or representatives from certain projects to
 participate in a non-voting capacity.