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: 
You'd also need a `idx.Write()`; otherwise the on-disk index won't reflect this change and after the commit, it will show up as deleted in staging.