contributing.md: mention rebased or squashed merges

.github/CONTRIBUTING.md

@@ -55,7 +55,7 @@ Follow these steps to backport a change into a release branch in compliance with
 1. Take note of the commits in which the change was introduced into `master` branch.
 2. Check out the target _release branch_, e.g. `release-20.03`. Do not use a _channel branch_ like `nixos-20.03` or `nixpkgs-20.03`.
 3. Create a branch for your change, e.g. `git checkout -b backport`.
-4. When the reason to backport is not obvious from the original commit message, use `git cherry-pick -xe <original commit>` and add a reason. Otherwise use `git cherry-pick -x <original commit>`. That's fine for minor version updates that only include security and bug fixes, commits that fixes an otherwise broken package or similar.
+4. When the reason to backport is not obvious from the original commit message, use `git cherry-pick -xe <original commit>` and add a reason. Otherwise use `git cherry-pick -x <original commit>`. That's fine for minor version updates that only include security and bug fixes, commits that fixes an otherwise broken package or similar. Please also ensure the commits exists on the master branch; in the case of squashed or rebased merges, the commit hash will change and the new commits can be found in the merge message at the bottom of the master pull request.
 5. Push to GitHub and open a backport pull request. Make sure to select the release branch (e.g. `release-20.03`) as the target branch of the pull request, and link to the pull request in which the original change was comitted to `master`. The pull request title should be the commit title with the release version as prefix, e.g. `[20.03]`.
 
 ## Reviewing contributions