Support AsciiDoc includes

When processing the README.adoc for the start-page of a project, all includes (of files within the repository) should be performed.

This allows you to move the content to a folder that is observed by Gradle's countinuous build (see: https://github.com/asciidoctor/asciidoctor-gradle-plugin/issues/192).

Currently only a link to the content is shown on the start-page when the README.adoc contains e.g.:

// The Content of README.adoc needs to be in the src-folder, since otherwise
// the continuous build appears to be impossible.
// https://github.com/asciidoctor/asciidoctor-gradle-plugin/issues/192
include::src/docs/asciidoc/README.adoc[]

In my opinion showing only a link to the included file is the correct behaviour when browsing the files. But displaying the README.adoc on the start-page is a special case, since it should provide the visitor with all information - even if they are outsourced to a different file.