Create CODE_OF_CONDUCT.md for r-devel repos

[EllaKaye]

Note that this is a separate task to creating a Code of Conduct for The R Project.

Once that is done, it could supersede the r-devel org CoC, but we could probably create and approve an r-devel org CoC sooner than one for the The R Project as a whole.

[russHyde]

Please see https://github.com/r-devel/template-repository/blob/main/CODE_OF_CONDUCT.md