Missing CODE_OF_CONDUCT.md

[ryohey]

CODE_OF_CONDUCT.md linked in CONTRIBUTING.md does not exist in main branch.
It was removed in the PR #2157, but was not intended?.
CODE_OF_CONDUCT.md is added at 5d05d97

[dcodeIO]

This file has meanwhile been moved to the dedicated top-level organization repository (here). It's linked from the repo itself on the right sidebar, but seems we missed updating it in CONTRIBUTING.md.