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 
We should put the whole range that we support so the upper bound is clear.

* Ruby 1.8.7 - 2.1.2

Asciidoctor 1.5.0 is the last release that supports Ruby 1.8.7. We'll likely still support Ruby 1.8.7 for all 1.5.x releases, but not 1.6.0.