update README to incorporate into asciidoctor.org
update README to incorporate into asciidoctor.org

- hide status except when viewed on GitHub
- show icons when viewed GitHub
- enable toc when viewed on GitHub or through browser preview
- hide contributing section except when viewed on GitHub
- use incremental subs to enable attributes in source listings
- fix capitalization of some section titles
- move compatibility section to appendix
- rename section titles under custom extensions section for clarity

asciidoctor/asciidoctor-gradle-plugin,asciidoctor/asciidoctor-gradle-plugin,asciidoctor/asciidoctor-gradle-plugin,ysb33r/asciidoctor-gradle-plugin,ysb33r/asciidoctor-gradle-plugin,ysb33r/asciidoctor-gradle-plugin,ysb33r/asciidoctor-gradle-plugin,ysb33r/asciidoctor-gradle-plugin,asciidoctor/asciidoctor-gradle-plugin,asciidoctor/asciidoctor-gradle-plugin