Improve library summary and quick start section

[thomashoneyman]

Is your change request related to a problem? Please describe.
As described in the documentation section of the Library Guidelines, Contributors libraries are expected to have in their README a short summary of the library's purpose, installation instructions, a quick start with a minimal usage example, and links to the documentation and contributing guide.

The library has a summary and quick start in the README, but they read almost like minimal reference documentation and deserve some love.

Describe the solution you'd like
The library summary should, in a couple sentences, motivate why this library exists. Right now the description just states it's an alternative to moment.js, but we should really add a couple sentences about what this library allows you to do (beyond a reference to a JS library).

The quick start really ought to demonstrate how to create and use some common formatters, like a number and a date, both via parsing string formats or via constructors (as described in #22).

The current quick start could be the start of more fleshed out documentation in the docs directory.

Additional context
See the Governance repository for more information about requirements in the Contributors organization.