Add code of conduct (#6875)

This code of conduct lives in <https://github.com/ocaml/code-of-conduct>
and has been discussed [in this
thread](https://discuss.ocaml.org/t/ocaml-community-code-of-conduct/10494).
It has been adopted in ocaml/ocaml in ocaml/ocaml#11761, and after a
discussion between project maintainers it is now enabled in dune as
well.

Signed-off-by: Etienne Millon <[email protected]>

Signed-off-by: Etienne Millon <[email protected]>

Author: emillon

CODE_OF_CONDUCT.md

@@ -0,0 +1,7 @@
+# Code of Conduct
+
+This project has adopted the [OCaml Code of Conduct](https://github.com/ocaml/code-of-conduct/blob/main/CODE_OF_CONDUCT.md).
+
+# Enforcement
+
+This project follows the OCaml Code of Conduct [enforcement policy](https://github.com/ocaml/code-of-conduct/blob/main/CODE_OF_CONDUCT.md#enforcement).