Update this repo's README

The updated README should be updated to contain:

• the scope of the web site it supports (asciidoc-wg.eclipse.org vs. asciidoc.org)
• Ammend authors of the repo (as possible contacts for people to ask questions) - adding different roles to different people; for example Guillaume and Alex maintaining content, Chris creating the original template and infrastructure setup
• Adding AsciiDoc as a trademark to the list