The files contained in src/main/asciidoc are asciidoc files. HTML / PDF can be generated out of them using the asciidoc tool, or by running mvn generate-resources. After Maven runs, you can read the HTML documentation by starting at target/generated-docs/index.html.

The original content was lost when the code was moved from codehaus.org over to GitHib. Thanks to web archive, it was possible to get the original content from e.g. http://web.archive.org/web/20150520175151/https://docs.codehaus.org/display/BTM/Home.