Create CODE_OF_CONDUCT.md

[FMCalisto]

Fixes # .

Description

A few sentences describing the changes proposed in this pull request.

Status

Ready/Work in progress/Hold

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh --codeformat --coverage.
• Quick tests passed locally by running ./runtests.sh --quick.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.

[wyli]

thanks! could you sign the commit (https://github.com/Project-MONAI/MONAI/blob/master/CONTRIBUTING.md#signing-your-work) ? also please fill the doc with these meta data if needed: https://github.com/Project-MONAI/MONAI/blob/master/setup.cfg#L2

[wyli]

this pr adds a default code of conduct document based on https://github.com/github/opensource.guide#readme