Missing contributing and code of conduct files

The readme [references](https://github.com/o19s/ubi/blob/main/README.md?plain=1#L130) contributing and code of conduct files that do not exist.

* https://github.com/o19s/ubi/blob/main/CONTRIBUTING.md
* https://github.com/o19s/ubi/blob/main/code_of_conduct.md

