Jp/doc

Here are some updates to the documentation. This adds some introductory material: the bare amount needed to get started with `gitlib`. Unfortunately, I haven't figured out how to use it yet, so **this is quite incomplete**.

*This isn't ready* for a pull request (at least, not to `master`). Could you please educate me, or update the documentation with something to get me (anybody) started?

Things that are (sorely) **lacking** from this branch:

* What is `MonadGit`, and how do I use it?
* Do I *really* need to create my own instance of it? If so, how? (I'm assuming there's a relatively simple way, some wrapper around the `IO` monad or such.)

* That `TreeBuilder` API through a `State`-threaded monad looks really cool, especially compared to what [libgit2] offers at the low-level. Please "brag" about that some; things like that are the reason we're using Haskell in the first place!

[libgit2]: http://libgit2.github.com/docs/guides/101-samples/#trees_treebuilder