add mention of pre-commit to contributing

docs/source/contributing.mdx

@@ -1,6 +1,13 @@
 # Contributors guidelines
 ... stil under construction ... (feel free to propose materials, `bitsandbytes` is a community project)
 
+# Setup pre-commit hooks
+- Install pre-commit hooks with `pip install pre-commit`.
+- Run `pre-commit autoupdate` once to configure the hooks.
+- Re-run `pre-commit autoupdate` every time a new hook got added.
+
+Now all the pre-commit hooks will be automatically run when you try to commit and if they introduce some changes, you need to re-add the changed files before being able to commit and push.
+
 ## Documentation
 - [guideline for documentation syntax](https://github.com/huggingface/doc-builder#readme)
 - images shall be uploaded via PR in the `bitsandbytes/` directory [here](https://huggingface.co/datasets/huggingface/documentation-images)