Some notes about libgit2/rugged

I learned libgit2/rugged when I involved in a project which is to analysis a repository. I found it's a little hard to use Rugged to do the basic things, e.g. `git pull`, `git push`, etc. This is the purpose that I want to add some notes here. In case of someday there's someone likes me who encounter into these kinds of stuff. ლ(╹◡╹ლ)