added explanation for dynamic rendering

[johannes-el]

This PR tries to resolve #168 by introducing an explanation for dynamic rendering in the overview section of the tutorial.

[SaschaWillems]

See #190, we should redo the whole chapter.

[johannes-el]

@SaschaWillems Yes you are right. But I thought doing it in little pieces is a better way to resolving this.
I can work on the other points mentioned in #190, sure.

[gpx1000]

Thanks for doing the dynamic rendering portion. I didn't want to lose your changes while we work on the other parts of #190. If you get to updating the other parts before we do, then we'll be grateful for any changes you suggest there.