Adopt the OCaml Code of Conduct

[rikusilvola]

The OCaml Code of Conduct can be found in ocaml/code-of-conduct and has been discussed in this Discourse thread.

We propose adopting it for ocaml-community/utop as well.

Once adopted, the list at ocaml/code-of-conduct needs to be updated.

• [x] #436
• [x] Update List of Adopters https://github.com/ocaml/code-of-conduct/pull/14

[emillon]

Thanks, I merged and created a PR to the list.