Add link for Marketing Working Group Mailing List

Created by: kevin-mcdermott

The marketing working group description does not include the details and link to join the e-mail list. Can you add the following text and hyper link in the same style as the others.

https://www.openhwgroup.org/working-groups/

You can join the OpenHW MWG Mailing List here. https://accounts.eclipse.org/mailing-list/openhw.mwg