doc: add review guidelines for collaborator nominations

GOVERNANCE.md

@@ -183,6 +183,34 @@ Collaborators might overlook someone with valuable contributions. In that case,
 the contributor may open an issue or contact a collaborator to request a
 nomination.
 
+#### How to review a collaborator nomination
+
+A collaborator nomination can be reviewed in the same way one would review a PR
+adding a feature:
+
+* If you see the nomination as something positive to the project, say so!
+* If you are neutral, or feel you don't know enough to have an informed opinion,
+  it's certainly OK to not interact with the nomination.
+* If you think the nomination was made too soon, or can be detrimental to the
+  project, share your concerns, ideally before the public nomination is opened,
+  and avoid sharing those concerns outside of the Collaborator discussion area.
+  Ideally, list what step(s) the nominee could take that would make you
+  approve their nomination.
+  Given that there is no "Request for changes" feature in discussions and issues,
+  try to be explicit when your comment is expressing a blocking concern.
+  Similarly, once the blocking concern has been addressed, explicitly say so.
+
+Our goal is to keep gate-keeping at a minimal, but it cannot be zero since being
+a collaborator requires trust (collaborators can start CI jobs, use their veto,
+push commits, etc.), so what's the minimal amount is subjective, and there will
+be cases where collaborators disagree on whether a nomination should move
+forward.
+
+When concerns have been raised on the private discussion, refrain from opening
+the public issue. If no one has explicitly blocked the nomination and you'd like
+it to move forward, comment something like "If I don't hear any objections
+before (some time), I will open the public issue".
+
 ### Onboarding
 
 After the nomination passes, a TSC member onboards the new collaborator. See