Split up example functionality a bit more

Just separating PubNub initialization from channel subscription so they can be
separate steps in the README.
Indicate clearly where you can type pageload code

It might be confusing if that gets printed to the chat window or JS console.
Add self-serve walkthrough for building a client

This can serve as a guide for kids as they go through the lesson, or for anyone
else joining in or learing in the future. Every step is pretty bite sized,
though there's a lot of hand waving going on.
Embiggen textarea text

Matching everything else and making it easier to read.