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
Replace '... if not...' by '... if they are not...'
Add questions for the review
Keep the URIs DRY and add suffix -uri
Reword the README

 * Keep the URIs dry
 * Replace "render" with "convert"
 * "Upgrading" is now a sub-section of "Installing"
 * Move the sentences that encourage contribution in the "Contributing" section
 * Rewrite the "Using custom templates" section
 * Add a note on the benefit and downside of using Linux package in "Installing" section
Reword the README

 * Adds an image of Asciidoctor on Asciidoctor
 * Remove resolved questions
revisions; update screenshot

- reword intro
- mention AsciidoctorJ and Asciidoctor.js
- fix numbered list
- reword Authors section
README revisions

* trim down API section
* new screenshot
* add Asciidoctor Project to copyright
* cleanup attribute names for uris
* copy edits
* AsciiDoc formatting
optimize access to attributes in Document#save_attributes method
update CHANGELOG.adoc with changes in 1.5.0
Merge pull request #4 from mojavelinux/pr-1001-revisions

README revisions
update changelog
follow-up to #574 support multiple values in single -r or -I flag
update comment
only warn, don't drop when attribute-missing is warn
allow syntax to be specified as positional parameter of stem block
slight tweak to spacing in header
ignore generated html files
minor optimization
don't match adjacent superscript and subscript chars