Add contributing guidelines

Regulate git commit style and ban LLM usage.

Reviewed-by: Aleix Roca Nonell <aleix.rocanonell@bsc.es>
Reviewed-by: Aleix Boné <abonerib@bsc.es>

README.md

@@ -7,3 +7,5 @@ This repository provides two components that can be used independently:
 
 - NixOS configurations for jungle machines. Use `nixos-rebuild switch --flake .`
   to upgrade the current machine.
+
+Before contributing, please read the [guidelines](CONTRIBUTING.md).