README.md: compute composite in a new image

The previous invocation did it in place, ovewriting the sinusoid image.
README.md: display images using the ImageJ UI

This was previously not possible due to a bugs in ImageJ.

But those bugs has now been fixed:
* https://github.com/imagej/imagej-common/commit/c99b6d3feb72dfd3ffe0322d4c29ddf2053c385e
* https://github.com/imagej/imagej-legacy/commit/2d6a8c52ad5219e6bda585ed9e63410471b5f815

So let's visualize the beautiful results of OPS in all their glory.
Merge branch 'readme-images'

Use real images when displaying the results, instead of ASCII.