Write up proposals repo etiquette document.

[TrevorFSmith]

Rough draft:

• on creating a repo, send group email about its creation
• proposal repos should ideally but not necessarily have two admins from two orgs
• there is a bootstrapping period with self-reviewed and merged PRs during initial setup and until other interested parties sign up for review
• then PRs that are reviewed and signed off by someone from another org

[himorin]

@AdaRoseCannon @cwilso no need to keep this issue anymore?? (CG/WG chairs are now the same..)