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 
Let's use the convention of appending "-uri" to these attribute names. For example:

```
:issues-uri: https://github.com/asciidoctor/asciidoctor/issues
```

Also, we can now use attribute references inside of attribute entries to keep the URIs DRY:

```
:repo-uri: https://github.com/asciidoctor/asciidoctor
:base-help-uri: https://help.github.com/articles
:issues-uri: {repo-uri}/issues
:fork-help-uri: {base-help-uri}/fork-a-repo
:branch-help-uri: {fork-help-uri}#create-branches
:pr-help-uri: {base-help-uri}/using-pull-requests
:gist-uri: https://gist.github.com
```