Code of conduct document

While reviewing project metrics for a grant application we realized that although we have a common understanding about how to conduct operations in the Slicer community, we do not have a formalized document to describe it.

The Contributor Covenant is a short and clear description that captures well how we operate, so we propose to adopt this as code of conduct document for Slicer.

If you have any questions, comments, or suggestions, please add a comment to this pull request till Wednesday.

To follow up on this, consensus was reached :100: :pray: :tada:

See Slicer/ and its history in git.

The pull request PR-5645 has been updated to first copy the v2.1 of the covenant and then introduce the Slicer specific changes.

More specifically commit Slicer@d6ab827ab (DOC: Adapt the code of conduct removing “Enforcement Guidelines” )