Update Repository Section of README for clarity

I found the README to be slightly confusing so I thought I might propose a few changes. 

As the README is currently written it seems like the target of  the `repo.head` object is a SHA. In reality, the target of  the `repo.head` object is a `Commit` object instead of a SHA. I've updated the documentation to attempt to clarify this. 

I've also added instructions for the case where someone _does_ want the SHA instead of just the `Commit` object.