Establish a code of conduct

We should establish a code of conduct for the project/repo.

Should we adopt the same code of conduct from the keylime/keylime repo [1]?

[1] https://github.com/keylime/keylime/blob/master/CODE_OF_CONDUCT.md

Review the proposed keylime/keylime CODE_OF_CONDUCT.md linked in the issue and compare it with the project’s current repository contents. Confirm whether that policy is suitable for rust-keylime, then add the agreed code of conduct file. Done means the repository contains the adopted policy and the project’s decision is clear.

Written by the indexing model from the issue text.