Establish governance docs for working groups

[earth2marsh]

With the Workflows specification reaching a 1.0 release, it will outgrow the "Special Interest Groups" moniker. This issue is meant to track the updates required to various governing documents either within or outside this repo.

1. Add/update docs for SIGs to explain how they evolve [either become a Working Group (Workflows), fold back into the spec (potentially Overlays), disband (?), or continue in perpetuity (Security never ends)]
2. Outline any mechanisms necessary to establish and describe Working Groups
3. Identify any related areas that might benefit from clarification, such as the description of the TOB.