Update quickstart instructions with `new` command

Quickstart instructions as written would serve an empty directory.

They instruct the user to install Jekyll (:metal), `cd` to a directory that may not exist, and then run `jekyll serve` (:hurtrealbad:).

Instead, lets have them run `jekyll new` to stand up a scaffold site, so they can see how things work and poke around a bit.

It's one more command, but IMHO a better user experience for someone just getting started.
Merge pull request #966 from benbalter/quick-start-scaffold

Update quickstart instructions with `new` command