convert README from rdoc to md

using

   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) }

as suggested in https://github.com/sinatra/sinatra/pull/595