Fork the second edition into the 2018 edition

[steveklabnik]

It's that time! We cannot yet modify chapters the introduction, chapters 1, 10, 18, 19, and 20, nor the appendices, But that's close enough to get going.

A few notes for you, Carol:

1. check out the CONTRIBUTING changes; I'm basically declaring the first edition totally frozen, and cleaning up the info on the second edition to say "hey we can make some small changes but that's it" and dropping the other stuff.
2. I tweaked the book index slightly. Accepting this PR doesn't mean that the 2018 edition is being published anywhere yet, so we don't yet mention it there. It's not totally clear when we'll want to; probably after we can modify all chapters.
3. Given the contribution stance, I modified them in all three books to be the same. This means that the intro for second edition got tweaked slightly; let me know if you want me to revert that or if this is okay.
4. the 2018 edition is currently 100% identical to the second edition with the exception of the source link. Actual work will start after we land all this.

[steveklabnik]

We might want to refactor the tooling out so that we use one copy of the tool for both editions, but i feel like that isn't urgent and can be a follow-up

[carols10cents]

This all looks great to me! There are a few typos in some of the new contributing text, but those can be fixed later :)