Convert README to Markdown.

Markdown formatting provides minimum hindrance when read in plaintext while
making this document much easier to comfortably read and parse in a browser.

Summary of changes:
- converted header formatting to Markdown
- converted long lists with `::` delimited key/values to Markdown tables
- homogenized inline code formatting
- used GFM code blocks with language-specific syntax highlighting
- fixed spelling of RSpec ("rspec")