Add links to Mailing Lists of the working group

There are at the moment two mailing lists (see https://accounts.eclipse.org/mailing-list).

Visitors see this as a first and low entry barrier way to participate.

name	description
asciidoc-lang-dev	AsciiDoc developer discussions
asciidoc-wg	This list is used to discuss the vision, scope, and governance of the AsciiDoc Working Group