Convert readme to markdown

[fhightower]

Sorry, this is a rant:

• I strongly dislike restructured text.
• I'm unable to deploy the readme in this package to pypi because:

HTTPError: 400 Client Error: The description failed to render in the default format of reStructuredText. See https://pypi.org/help/#description-content-type for more information. for url: https://upload.pypi.org/legacy/

and I don't have the time or patience right now to worry about it.