GOVERNANCE: Require explicit sponsor votes

To avoid uncertainty like [1].

[1]: https://groups.google.com/a/opencontainers.org/d/msg/dev/5qj2hATVxew/-ljDGQB0AQAJ
     Subject: Re: [runtime-spec VOTE]: Tag d3c3763b as v1.0.0-rc2 (closes
       2016-09-08 13:47 UTC)
     Date: Thu, 8 Sep 2016 13:48:38 -0400
     Message-ID: <CANcsyS6q5neOqT+MLxhFDvDKPLGaYpY=HKy0p4pMjs3zexQs_A@mail.gmail.com>

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

GOVERNANCE.md

@@ -15,6 +15,7 @@ A maintainer SHOULD propose a motion on the [email protected] mailing list
 Voting on a proposed motion SHOULD happen on the [email protected] mailing list (except [security issues](#security-issues)) with maintainers posting LGTM or REJECT.
 Maintainers MAY also explicitly not vote by posting ABSTAIN (which is useful to revert a previous vote).
 Maintainers MAY post multiple times (e.g. as they revise their position based on feeback), but only their final post counts in the tally.
+The sponsor(s) MUST explicitly vote to be counted; proposing a motion without an explicit LGTM does not count as a vote (but the sponsor MAY LGTM/REJECT the motion in their proposal email).
 A proposed motion is adopted if two-thirds of votes cast, a quorum having voted, are in favor of the release.
 
 Voting SHOULD remain open for a week to collect feedback from the wider community and allow the maintainers to digest the proposed motion.