Extending mainpage example

I've looked a bit into `daofind` and while I still don't understand it in detail, I'm pretty sure passing a `threshold` below the background level doesn't make sense. So could you please change the `-` to `+` here even if you loose some faint detections? I think it's OK to not detect the bright saturated stars if you explain that in a sentence, but if you do want them here I think you have to adjust the `sharp` and `round` cuts when calling [daofind](http://photutils.readthedocs.org/en/latest/api/photutils.daofind.html).