Some notes about libgit2/rugged

I learned libgit2/rugged when I involved in a project that is to analysis a repository. I find it's a little hard to use Rugged to do the basic thing, e.g. `git pull`, `git push`, etc. So I want to add some notes here. In case someday there's someone likes me who wants to search some useful infomation. ლ(╹◡╹ლ)