Add research usage intro paragraphs

scikit-image should be very attractive to researchers in image processing! Instead of jumping straight into examples, I hope these two new paragraphs will express some of the most salient reasons why scikit-image should be compelling to investigators in this field. I also made a couple of minor changes for directness and clarity.

On my Mac, a 70% scaled figure was extending across columns and occluding text in the opposing column; after reducing this to 60% it rendered correctly. If this breaks rendering for other platforms, I'll revert and investigate further.