Update the README's How it works section with a new structure

[consideRatio]

Proposed change

To update our README's "How it works" section according to @yuvipanda's suggestion.

1. What factors are taken into consideration for a notebook to be 'culled'. This would talk about CHP activity and when it is triggered, and same for the reports from the user server. This would be a high level description, and helps answer the question of 'What causes a notebook to be culled or not culled?'
2. How these factors are calculated technically. This would point to code and add more details as you have done.

While doing this, I'd like to ask that it remains relevant still under the category "How it works" as I've used it as a reference from z2jh's docs.