Add Git2go tutorial

This is an article about how to use [Git2go](https://github.com/libgit2/git2go) to work with git repositories.

It explain how to use the library to clone the gopheracademy repository, how to create a new branch, add a new commit with a markdown article and how to push it to your own fork. I personally used the code in this article to publish this article in my own fork :laughing: 
> Git2go uses a callbacks system to feed some processes with intermediate information.

It's not very clear to me.

Maybe something like:
> I'm going to use the ssh credentials in my machine to push the branch to my repository. Git2go uses callbacks to configure some of the processes with our own information. In this case, I'll need to provide two callback: one that connects with the ssh agent in my machine, and a second callback to verify that I'm connecting to GitHub: