added step to add sales people to sales@gitlab.com and a link to the gitlab email forwarding doc

@sytses @ernstvn

I added the step Sid mentioned in his email today. Sid called it sales@ forwarding but I do not think that is an email address so I called it sales@gitlab.com. Did I get that right?

Also added the link to the doc here - we can move it. Hope I entered that link correctly!