docs: richer README

[tiborsimko]

Let's enrich README to resemble other existing REANA examples such as:

• https://github.com/reanahub/reana-demo-worldpopulation
• https://github.com/reanahub/reana-demo-root6-roofit
• https://github.com/reanahub/reana-demo-bsm-search

This comprises several things:

• Explain astronomy behind the example to casual visitors: can be as brief or as detailed as deemed necessary. @volodymyrss Can you take care of that?

• Amend the REANA.rst structure following usual examples. This is just cosmetics, we could take it separately once the astronomy content is clear.

• The workflow produces two figures, good_fit_png_content.png and next_good_fit_png_content.png. Save them into docs/... folder so that people can compare. Use them in README for graphical appeal.