[template] Provide text for potential deliverables that might advance from incubation

[jyasskin]

Working groups have started adopting the pattern of listing deliverables that they might produce, in addition to the section in the charter template that lists specifications they "will deliver". The template should give example text for doing that and any constraints on how precise the description ought to be.

cc/ @tantek

This is related to #262 but doesn't need to be based on a relationship with a specific CG.