Add code of conduct

[gatheluck]

Why

A code of conduct is recommend to add by Open Source Guides.

Definition of Done

CODE_OF_CONDUCT.md is added to the project root.

How

• Create a draft of code of conduct.
• Share the draft to XCCV developer members and reach agreement.
• Write CODE_OF_CONDUCT.md.

Reference of code of conduct:

• hydra
• pytorch
• poetry