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: 
> and the tree where the index is pointing to to create this commit

sounds a bit weird.

Maybe rephrase to:
> To create this commit, we're going to use some from the branch we just created and the tree the index is pointing to. We'll also reuse my previous signature as a committer and author.