Readme rdoc to markup conversion

*Note: This is not a final PR*

I have converted the README.rdoc to Markdown via rdoc:

    require 'bundler'
    gem "rdoc", "~> 4.0.0.preview2.1"

    require 'rdoc'
    require 'rdoc/markup'
    
    h = RDoc::Markup::ToMarkdown.new
    file = File.read "README.rdoc"
    out = h.convert(file)

    File.open('README.md', 'w') { |file| file.write(out) }

So this is a rather simple conversion path but it has a couple of issues:

* easy so solve: Links are output as `{DSL}[http://en.wikipedia.org/wiki/Domain-specific_language]` (fixed in preview 3 of Rdoc)
* definitions don't exist in Markdown
* tables don't exist in Markdown

The links can easy be fixed by hand (filed a bug report at rdoc), the latter two need to be resolved with an alternative style. Any ideas?

EDIT: I added the first couple of lines since ToMarkdown is an Rdoc 4 feature and preview 3 is not yet released which has a bug fix for the broken links.