docs: try to clarify that we want contributors to squash fixup commits

It's happened a few times that contributors misunderstood the
guidelines, so they're clearly not clear enough. Hopefully this
clarifies.

docs/contributing.md

@@ -27,7 +27,8 @@ information on using pull requests.
 
 Unlike many GitHub projects (but like many VCS projects), we care more about the
 contents of commits than about the contents of PRs. We review each commit
-separately, and we don't squash them when the PR is ready.
+separately, and we don't squash-merge the PR (so please manually squash any
+fixup commits before sending for review).
 
 Each commit should ideally do one thing. For example, if you need to refactor a
 function in order to add a new feature cleanly, put the refactoring in one