Add contributing guidelines #20

Add a simple CONTRIBUTING.md document with general guidelines on what a developer should do before/during a new contribution to the repository. Simple things like "check if there's an existing issue or create a new one", "open a PR that has a good description and that links to the issue it closes", "Make sure the CI is passing"...

Examples:
Dashboard: https://github.com/kernelci/dashboard/blob/main/CONTRIBUTING.md
Kci-dev: https://github.com/kernelci/kci-dev/blob/main/CONTRIBUTING.md