Normal GitHub etiquette is to fork, make changes in your repository and then - if the changes are generally applicable to other users - submit a “pull request” to the original author.
You don’t need to document anything and you should not be shy about your coding abilities.
A slight snag is they’ve chosen the “No derivatives” version of the CC license, which prohibits distributing modified copies. Taken strictly this might apply to committing changes to a public fork, but they appear to have accepted pull requests from other forks so they obviously don’t care. *shrug*