Add a Code of conduct

[Chaves2021]

Document recommended by project's Community Standards

• Definition by code of conduct by open source guide: A code of conduct is a document that establishes expectations for behavior for your project’s participants. Adopting, and enforcing, a code of conduct can help create a positive social atmosphere for your community.

• Objective of the document by open source guide: Facilitate healthy and constructive community behavior by adopting and enforcing a code of conduct.

More infos about the document: https://opensource.guide/code-of-conduct/

Related to https://github.com/courselab/snaskii22/issues/58

[Chaves2021]

This model of code of conduct needs a contact to receive reports of any unacceptable behavior, if occurs.

@monacofj I would like to suggest you as contact to receive reports of any unacceptable behavior described in the document. What do you think?