Critique proposal

Here's a first draft on how we might do critique at 18F. I think this is a good starting point, but it's certainly not perfect -- just a conversation starter.

Happy to have conversation in this pull request, as well as in the design huddle. But I'd like to enshrine as much of it in this thread as possible so we can reference it in the future.

Everybody should comment, but I especially want to hear from @quepol @jtag @jehlers @russu