Add CONTRIBUTING guidelines

[AlessandroPierro]

User story

Add contributing guidelines for community developers: https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors .

Conditions of satisfaction

• An optional list of conditions that have to be fulfilled for this feature to be complete.
• For example: "Users can add both individual items and lists as parameters."

Acceptance tests

• An optional list of tests that should be written to automatically test the new feature.