Update README to document the new package layout and how to use it

[NoahAndrews]

No description provided.

[LandryNorris]

I plan to list the packages as a numeric list. The markdown spec allows numeric lists to be specified with a 1. at the start of each line. I generally prefer this, since it allows us to add items to the middle of the list without having to commit a change for every following item. IntelliJ-based IDEs typically warn about this, since they can automatically update the numbers. Most people will be viewing a README via GitHub's interface or an IDE that should recognize markdown and automatically show the right numbers.