Firstly, this is a little thing that is, at least initially, unlikely to have much call for additional contributions. Once it is achieved, it won't have much need for expansion. Nevertheless, it seems that every project needs to have some kind of contribution guidelines these days; so let's get this out of the way from the outset.
Essentially this boild down to: don't be a dick.
Perhaps the best, slightly more detailed Code of Conduct to adopt is the Python Software Foundation's Code of Conduct.
This project, like all of my projects, does not incorporate the Diversity Statement pushed within part of the Python community, nor does it include any version of either the Community or Contributor Covenants and for the same reasons. This is in accordance with my Project Participation Policy.
The short version as to why not adopt those viral CoCs is that they're all discriminatory against one particular minority and are thus totally dickish.