[Introductions] Add etiquette for double opt-in introductions.

[mko]

I noticed that in 9.md, your Great Introduction skips straight into the introduction. One of my biggest complaints about introductions I've received in the past is having the introductions not be double opt-in. I'd say currently that 9.md is a Good Introduction and that a Great Introduction would follow Fred Wilson's rule about double opt-in introductions:

When introducing two people who don't know each other, ask each of them to opt-in to the introduction before making it.

It works remarkably well, and I find that when you provide some private context for your reason for wanting to make the introduction, everyone starts off on better footing.

[dweekly]

This is a very reasonable point, and I quietly assumed it had already happened in 9.md.

I'll explore spelling this out in more detail as well as including good and bad "request for introduction" scenarios.