Document the different level of customization

[ggrossetie]

This documentation should cover the following items:

• Stylesheet (HTML5 output)
  • Use a theme (stylesheet): https://github.com/asciidoctor/asciidoctor-stylesheet-factory
  • Modify/override the default stylesheet
  • Create your own stylesheet: https://github.com/asciidoctor/asciidoctor-stylesheet-factory
• Create an extension
  • Extend the AsciiDoc syntax with a custom block/macro
  • Use a post processor to modify the output
  • Use a docinfo processor to insert content in the header or footer
• Custom templates
  • Provide a custom template to override the rendering of an AsciiDoc element
• Create a converter
  • Develop a new converter using the API

I think it should be ordered with a progressive level of difficulty and for each item we should have a "skill requirements". For instance, if you want to create a converter, you must at least know a programming language and the Asciidoctor AST.

[mojavelinux]

cc: @graphitefriction