Convert README to Markdown.

Convert README to Markdown.

This is a first step toward modernizing our README file.  Some
popular developer platforms support rendering README.md files, so
a direct conversion to Markdown seems like a good place to start.
The intent is to keep this file legible as plain text even as it
accumulates more content.

Suggested-by: Andrew Atkinson
Reviewed-by: Tom Lane, Daniel Gustafsson, Joe Conway
Discussion: https://postgr.es/m/CAG6XLEmGE95DdKqjk%2BDd9vC8mfN7BnV2WFgYk_9ovW6ikN0YSg%40mail.gmail.com