Update CONTRIBUTING.md

I keep repeating that commit messages should explain the behavior change and why, don't create merge commits, test and code changes should be made together (atomic), tests should pass at each commit (so future git bisect is possible).

CONTRIBUTING.md

@@ -43,7 +43,24 @@ the [puppet-dev mailing list](https://groups.google.com/forum/#!forum/puppet-dev
   * To quickly create a topic branch based on the default branch, run `git
     checkout -b fix/main/my_contribution main` (or `master` for older repos).
     Please avoid working directly on the default branch.
-* Make commits of logical and atomic units.
+* Make commits of atomic units.
+  * Your commit message should clearly describe how the behavior is changing and why.
+  * Commit messages that say "fixes bug" or are empty will be rejected.
+  * If code changes cause tests to fail, then the tests should be updated in the
+    same commit as the code changes.
+  * Squash intermediate commits.
+  * Do not create merge commits in your pull request. Instead rebase your pull
+    request. For example, if your feature branch tracks `upstream/main`, then
+    run `git rebase $(git merge-base HEAD upstream/main) --onto upstream/main`
+    and force push to your remote `git push --force origin HEAD`.
+* Make commits of logical units.
+  * Significant whitespace-only changes should be made in a single commit, not
+    comingled with non-whitespace changes.
+  * For significant changes, break down your work in separate commits to make it
+    easier to review.
+  * If your pull request contains multiple commits, then tests should pass at each
+    stage. To confirm, assuming your feature branch tracks `upstream/main`, then run
+    `git rebase -i $(git merge-base HEAD upstream/main) --exec 'bundle exec rake <task>'`
 * Check for unnecessary whitespace with `git diff --check` before committing.
 * Make sure your commit messages are in the proper format. We use the [50/72 rule](https://git-scm.com/book/en/v2/Distributed-Git-Contributing-to-a-Project) in commit messages.