Add community code of conduct

[Dynorat]

It is best practice to have a community code of conduct that sets clear expectations for participants in the community. This will help us be more aligned with general community best practices as defined here: https://github.com/IEA-Task-43/digital_wra_data_standard/community

I suggest referencing something from https://www.contributor-covenant.org/ or NUMFOCUS (https://numfocus.org/code-of-conduct)