Add README

As the README is prominently shown when a user enters this repository, I think it is important to have one.

The version coming with the initial merge request just holds the bare minimum.

I think we could add a lot of information here – basically the most important bits from the tutorial. As I don't have time for this right now, I'd suggest to at least merge this current minimal README right now, but keep the branch open in case anyone wants to augment the README (if that's at all possible… we'll see).