To contribute to code, you should first fork the containertree repository by clicking the fork button on the top right of the page. Once forked, you will want to clone the fork of the repository to your computer:

git clone<username>/container-tree
cd container-tree/

The main python module, containertree, is in the top level folder. You should checkout a branch, push the branch to your remote, and when you are ready, open a pull request against the master branch of singularityhub/container-tree.

git checkout -b add/my-feature
git commit -a -m 'adding my new feature!'
git push origin add/my-feature