We are happy you are interested in adding a tutorial, it is simple!
You must submit to the github repo a markdown file, we chose this way because then everyone becomes a community contributor and learns to collaborate in this manner.
Write your tutorial in markdown format
Join the Github organization, if you haven't already done so
Ask to contribute to the Github repository
Git clone the tutorial repository to your development machine
Make a new git branch, then switch to it
Add your markdown to the markdown folder and catalog
Submit a pull request after you are happy with the commit!