contributing: non-WIP should always be landable

I was recently surprised that we don't prefer addressing review comments
in 'fixup!' commits.

Coming from GitHub, I've found that fixup commits make it easier for
reviewers to see what has changed since their last review. The idea is
to use '--autosquash' before landing to clean up the history again.

It's a pleasant surprise that it's easy to keep track of what changed in
merge-requests between pushes, so we can always keep the history clean.

Document this, so folks like me can see the light sooner.

CONTRIBUTING.rst

@@ -128,6 +128,12 @@ If a commit in your branch modifies behavior such that a test must also
 be changed to match the new behavior, then the tests should be updated
 with the same commit, so that every commit passes its own tests.
 
+These principles apply whenever a branch is non-WIP. So for example, don't push
+'fixup!' commits when addressing review comments, instead amend the commits
+directly before pushing. GitLab has `good support
+<https://docs.gitlab.com/ee/user/project/merge_requests/versions.html>`_ for
+diffing between pushes, so 'fixup!' commits are not necessary for reviewers.
+
 
 Commit messages
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