Add example using CIAO tool fluximage

[hamogu]

While X-ray astronomers are used to event lists, users from other wavelength regimes might think in terms of images. For those users an example how to turn a marx simulation into an image might be useful. This example should at least mention some of the caveats while that's often not a good idea (arf and rmf change with detectors position, because PSF is energy dependent etc.)