Create structure around WG evaluation and "wind-down"

[nikomatsakis]

As a follow-up to #5, we need to think about the full lifecycle of domain working groups -- i.e., once they have started, how do we decide when it is time for them to stop? The core team recently decided to wind down the Async Ecosystem WG, for example, but we realized in the process we don't have much of a policy around this. So I'm opening this issue to reflect the need to form one.

Some things I think we could consider:

• Do we want to do some sort of regular evaluation? What are the criteria?
• Who makes the final decision? (Presumably same folks that decided to create)
• What are the "options" when winding down a working group, and can we lay out some of the possibilities? (e.g., maybe some working groups should become a full team, etc)