Make certain usage patterns more prominent in the README (#3557)

* Make certain usage patterns more prominent in the README

We use this library extensively where I work. After pairing with many
teammates, I've learned that one common stumbling point when using this
library is how to first get a JSON value in the first place (because
once they have a variable of type `json` in hand, the API is intuitive
and mostly does what you expect).

With that in mind, I've added two subsections to the top of the Examples
section: how to read JSON from a file, and how to hardcode a JSON
literal.

I understand that these are already documented elsewhere in the Examples
section, and so these new subsections are technically redundant. But to
defend this choice:

- Redundancy in docs is actually good, because not everyone consumes
  docs in the same way or in the same order.

- Having these things called out explicitly in isolation encourages
  people to get something working faster. In later sections, these
  examples are presented alongside many other options. Having to choose
  between alternatives gets in the way of quickly prototyping something.

If you have further suggestions for how to improve or modify these docs
I'm open to them. The changes included here would really make a huge
improvement in the productivity of newer members of my team using this
library.

* Remove old TOC entry

* Put the section back

* Another tweak