Add explanation text for navigation

[krisjpi]

Is it worthwhile adding some text explanation somewhere about moving around and zooming? We could probably cut some of the text in the earlier intro slides or add another slide at the end.

Or do we assume the controls are intuitive enough without explanation?

ex. "use the scroll wheel to zoom in and out", "click and drag to move"

Adding text to the background could work as an alternative to adding intro slides.

[krisjpi]

this might also be grouped with #149