Ensure each package's README lists purpose and accurate usage examples

[mcmire]

Most of the packages in the core repo have a very spartan README. While this makes it easy to maintain, it is not very helpful for contributors.

Ideally, each package's README should describe its purpose and provide usage examples.

As a bonus, we should consider finding a way to make the usage examples executable, and run them in CI (similar to Rust's documentation tests).