CONTRIBUTING: Mention not to open several pulls

CONTRIBUTING.md

@@ -87,6 +87,15 @@ before it will be merged. The basic squashing workflow is shown below.
     # save and quit
     git push -f # (force push to GitHub)
 
+If you have problems with squashing (or other workflows with `git`), you can
+alternatively enable "Allow edits from maintainers" in the right GitHub
+sidebar and ask for help in the pull request.
+
+Please refrain from creating several pull requests for the same change.
+Use the pull request that is already open (or was created earlier) to amend
+changes. This preserves the discussion and review that happened earlier for
+the respective change set.
+
 The length of time required for peer review is unpredictable and will vary from
 pull request to pull request.