add link to examples project
add link to examples project

- add link to examples project
- clarify Usage introduction
- mark recommended file extension
- minor rewording

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