Improve documentation

[JanBenisek]

Make documentation more precise, based from feedback of our users, these are the points they were missing:

• if X happens, do Y or that (example when variable is dropped when it has on 6 bins, but we set 10)
• be more explicit in what Cobra does and does not do
  • AUC is being optimized
  • no interaction variables
  • if model fails, try this or that
  • ...
• why it works
  • why logistic regression (and not decision tree)
  • why we bin continuous variables (we could keep them as they are)
  • why we do incidence replacement
  • link to papers/more fundamental research
• how some steps works
  • show on example how regrouping works
• add link to Geert's paper
• explain how preprocessing works
  • regrouping
  • binning

[sborms]

For the new release, there will be more documentation across the README page, the code functions, and the tutorials. Not yet fully optimal but a good step forwards. Most of your points are better covered.