Rework the README

 * Move contributing docs to CONTRIBUTING.adoc
 * Concise sentences to reduce README length
 * Move Contributing section below Usage
 * Merge Operating Systems section and Dependency and Configuration Requirements into Requirements

Resolves  #561 
All uses of the word "render" (in prose and APIs) should be replaced with "convert". Render is what the browser does. Convert is what Asciidoctor does.