GOVERNANCE.md: role-transitioning guidelines are not exhaustive

GOVERNANCE.md

@@ -71,6 +71,8 @@ and we will strive to **get barriers out of the way** of these contributions.
 To ensure that these role transitioning processes are
 straightforward, transparent, predictable, and impartial,
 the metrics used are objective, easy to check, and explicitly described below.
+(That's not to say they're hard-set rules:
+exceptions can always be considered, via open community discussion.)
 
 - Regular contributors shall be recognized as collaborators in the organization.