revise README
revise README

- enable additional settings for generation
- clarify recommended Ruby version
- include content of example.adoc using include directive

hmflash/asciidoctor-pdf,DavidGamba/asciidoctor-pdf,theimdal/asciidoctor-pdf,mojavelinux/asciidoctor-pdf,dsisnero/asciidoctor-pdf,Hextremist/asciidoctor-pdf,Hextremist/asciidoctor-pdf,hmflash/asciidoctor-pdf,asciidoctor/asciidoctor-pdf,dsisnero/asciidoctor-pdf,theimdal/asciidoctor-pdf,DavidGamba/asciidoctor-pdf,abatalev/asciidoctor-pdf,mojavelinux/asciidoctor-pdf,asciidoctor/asciidoctor-pdf,abatalev/asciidoctor-pdf