migrate README files from Markdown to AsciiDoc

Reasoning:

* Can use Asciidoctor.js Live Preview browser extensions (Chrome, Firefox) to view the rendered version without having to run any builds
* Consistent with the format used for documentation
* Offers better semantics for representing the information, most notably definition lists
* Renders better on GitHub

The changes are primarily a direct translation from Markdown to AsciiDoc. There are a handful of typing corrections thrown in, but nothing significant.