There are many ways that you can contribute to H5P, both closely related to source code (updating translations, creating content types, fixing bugs, etc.) and beyond (answering questions in the H5P forum, spreading the word at conferences, raising funds for new features, etc.). This repository with all its contents is intended to support the former.
There are several great resources already all over the internet, and instead of reinventing the wheel, we'll reference those here. Why here? Because github is the place where the H5P source code can be found and improved, and the same should be true for this documentation, tutorial or whatever you'd like to call it. You are invited to correct things, improve misleading information, translate the texts to other languages, etc. using pull requests. That's the same mechanism we're using to include code contributions from the community. So, why not start with learning the first steps of how to translate content types or adding new features by working with this repository -- and maybe tweaking it here on the go?
Of course, you're also welcome to use create some other things that can help to work with the documentation, e.g. tutorial videos, H5P content helping you to learn about creating H5P content types ;-), etc.