If you're using Git command line or any command line to update your repo on mac or linux machine, you can run the following commands to create the directory:

1. Navigate to the git repo location you want to add a new folder to in command line.
2. Run mkdir folder_name
3. Run ls to list folder changes which should show the new folder.
4. Add a new file to the directory. (You must add a file to the directory else git won't show anything to commit)
5. Next run git add folder_name that contains the new file(s).
6. git commit -m "With you message"
7. git push
8. Visit your repository on GitHub and notice that your changes are reflected there as well online.