Introduction to Handbook

[gcrev93]

(If you would like me to separate these into different issues I can. Just wanted to keep all notes for the page together)

[elibarzilay]

In addition, the very first sentence is a harsh CS-like talk about sets and behaviors, it should be much more friendly for people who are not familiar with static type checking... (I thought about creating a new issue, but it looks like it fits well with this one.)