Package Level READMEs

It would be really helpful to have a high level `README.md` for each package in the `lib` directory. These could contain a high level overview of the package, including the exports, the classes and their methods, what the package does and any other information that would make understanding the package easier for people. A good example could be found in [btcd](https://github.com/btcsuite/btcd), where they document each package with a `README.md`. See this link for an exact example: https://github.com/btcsuite/btcd/tree/master/mempool

Desired information in a `lib/{package}/README.md`:
- Classes/functions exported
- Class methods
- Minimal amount of configuration to instantiate a Class
- High level overview of the package itself

Feel free to chime in with any additional thoughts/opinions in what would make these `README`s more useful