Improve Readability (and Searchability) of README

[agilgur5]

• add a ToC
  • make each subsection of usage an actual subsection/subheading
• add a before/after of spaghetti code to using this?
• add more wordings
  • c.f. https://stackoverflow.com/questions/3753359/serializing-foreign-key-objects-in-django#comment92601787_49482337
  • one-to-many and many-to-many on top of "foreign key" as it's not always understood that these are similar (or the same in some cases).
    there's also the difference between the forward and reverse relation of ForeignKey -- one is select_related, the other is prefetch_related
  • select_related and prefetch_related used more as well as in section headings / ToC