sync README content with website; add info about Gradle build
sync README content with website; add info about Gradle build

- sync content from website
- add information about Gradle build
- explain how to perform a release

asciidoctor/asciidoctorj,ancho/asciidoctorj,robertpanzer/asciidoctorj,asciidoctor/asciidoctorj,abelsromero/asciidoctorj,ysb33r/asciidoctorj,mojavelinux/asciidoctorj,solent/asciidoctorj,ancho/asciidoctorj,ysb33r/asciidoctorj,danielgrycman/asciidoctorj,danielgrycman/asciidoctorj,abelsromero/asciidoctorj,lefou/asciidoctorj,lefou/asciidoctorj,danielgrycman/asciidoctorj,robertpanzer/asciidoctorj,solent/asciidoctorj,abelsromero/asciidoctorj,ysb33r/asciidoctorj,solent/asciidoctorj,lefou/asciidoctorj,danielgrycman/asciidoctorj,robertpanzer/asciidoctorj,lefou/asciidoctorj,ysb33r/asciidoctorj,mojavelinux/asciidoctorj,asciidoctor/asciidoctorj,mojavelinux/asciidoctorj,ancho/asciidoctorj,solent/asciidoctorj