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 
This whole section really needs to just go away. For one, it's not even that accurate anymore. I'd almost say we should just provide a simple example of a custom template (like a custom paragraph template), and then point them to the user manual where this is all explained.

For example, we can say, "Let's assume you want to customize the HTML that is generated for a paragraph. First, create the file _templates/paragraph.html.slim_ with the following content:

```
p id=@id class=role =content
```

Then, pass the `template_dir` option to the API to have Asciidoctor use this template for converting paragraph blocks in place of the built-in converter for paragraphs.