Adds additional responsibilities to the website working group

This might be a bit verbose as is, although I think the examples help clarify the intent.

• Reference to WG acceptance of "evangelism" efforts https://github.com/iojs/website/blob/master/wg-meetings/2015-01-19.md#matching-or-diverging-from-iojs-organizationally
• Social media and documentation were discussed/approved Feb 2 but the minutes from that meeting haven't been PR'd in yet. https://www.youtube.com/watch?v=SBJaXUA0lSY

Of course, pending general TC approval of the scope we've said we can take on / help develop.