This is kind of a bike shed, but at this point it's a little more clear
that Markdown has won the format wars and that people are a little more
used to seeing/editing it.
This moves the `README` to Markdown and also includes some minor
cleanup:
* Fixes a broken reference link (`curl`).
* Normalizes lines to 80 characters.
* Normalizes code snippets to be prefixed by four spaces.
* Normalizes sentence to be separately by only a single space.
