Updates extension section.
Updates extension section.

DocinfoProcessor section has been added in Extensions chapter. A simple example has been provided following Asciidoctor example.
ancho/asciidoctorj,solent/asciidoctorj,ysb33r/asciidoctorj,danielgrycman/asciidoctorj,danielgrycman/asciidoctorj,lefou/asciidoctorj,ysb33r/asciidoctorj,robertpanzer/asciidoctorj,robertpanzer/asciidoctorj,solent/asciidoctorj,lefou/asciidoctorj,abelsromero/asciidoctorj,robertpanzer/asciidoctorj,ancho/asciidoctorj,solent/asciidoctorj,asciidoctor/asciidoctorj,solent/asciidoctorj,lefou/asciidoctorj,abelsromero/asciidoctorj,ancho/asciidoctorj,danielgrycman/asciidoctorj,asciidoctor/asciidoctorj,abelsromero/asciidoctorj,lefou/asciidoctorj,ysb33r/asciidoctorj,asciidoctor/asciidoctorj,danielgrycman/asciidoctorj,ysb33r/asciidoctorj