chore(docs): explain that the repo uses conventional commits

This adds a new section to CONTRIBUTING.md that explains that we are
using conventional commits and that commithelper can be used to create
valid commit messages

CONTRIBUTING.md

@@ -1,5 +1,9 @@
 # Contributing
 
+## Making a commit
+
+This repository uses [conventional commits](https://www.conventionalcommits.org/en/v1.0.0/) to be able to generate a meaningful changelog automatically. After you make changes and added them e.g. by running `git add <my changed file>`, use `git commit` (without `-m`) to get help for creating a valid commit message. If you use `-m` (or write a commit message directly in your editor for example), your message will still be checked and the commit will fail if it is against the commit conventions.
+
 ## Making a release
 
 You will need a personal GitHub API token (this is used to create the release on GitHub). You can obtain one [here](https://github.com/settings/tokens/new?scopes=repo&description=release-it) (it only needs "repo" access, not "admin" or other scopes).