Create CODE_OF_CONDUCT.md

[Hoverbear]

Add the code of conduct used in our other projects. This is the Contributor's Code of Conduct, the content of the document should not be edited. The words were chosen carefully.

• https://github.com/pingcap/raft-rs/blob/master/CODE_OF_CONDUCT.md
• https://github.com/pingcap/grpc-rs/blob/master/CODE_OF_CONDUCT.md