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.