docs: add code of conduct

[haneenmahd]

proposes a new code of conduct for socket.io repo.

The kind of change this PR does introduce

• [ ] a bug fix
• [ ] a new feature
• [x] an update to the documentation
• [ ] a code change that improves performance
• [ ] other

Current behavior

Has no code of conduct.

New behavior

Has a code of conduct which sets community standards and rules for the repo.

Other information (e.g. related issues)