Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -53,7 +53,7 @@ In any case, it might be a good idea to take a short tour as described [here](./
 We are very happy if you decide to contribute code! We are committed to keep all contributions to vodle free and open-source software, licensed under the very strong copyleft license AGPL-3.0 or later. Hence we need all contributors to give use the necessary nonexclusive rights that will allow us to enforce this license. This is the reason why we ask you to sign a Contributor License Agreement (CLA) before we merge your pull request. We have chosen a very light and balanced version of CLA designed by the Free Software Foundation Europe, the so-called "Fiduciary License Agreement (FLA) 2.0" which was [designed to ensure that the software will forever remain Free Software](https://fsfe.org/activities/fla/fla.en.html) without giving us undue power. If you have any questions about this, please contact us.
 
 Before you start coding: 
-- Please set up your git to [sign all commits](https://docs.github.com/articles/about-gpg/) automatically, because our branch protection rules require all commits to be signed before merging them into main, and it is easier to do it right away than signing them afterwards.
+- Please set up your git to [sign all commits](https://docs.github.com/articles/about-gpg/) automatically, because our branch protection rules require all commits to be signed before merging them into main, and it is easier to do it right away than signing them afterwards. (In case you forgot to sign some commits before pushing them, you can follow [this guide](https://webdevstudios.com/2020/05/26/retroactively-sign-git-commits/) to sign then retroactively, but that requires more work than setting it up in advance).
 
 Once you do a pull request:
 - Please [sign our Contributor License Agreement](https://cla-assistant.io/pik-gane/vodle).