Adding Text to Static Analysis Chapter

I've added the beginning (intro sections) of the chapter. I'm going to add 4 more sections, one for each of three "checks" that TypeCheck can make.

There probably needs to be more "this is how this works in Julia" notes added to the early code examples.
It wasn't obvious to me right away that a variable with a type annotation could still be assigned something of a different type. 

E.g. the fact that x::Int64 is in the method signature, but that it can later be coerced to float was a surprise to me, and it wasn't until I'd processed that surprise that I truly understood what you meant by 'type stable'. (I suppose this is what happens when you want a dynamic feel to variable assignment, but you still need to state parameter types for multiple dispatch.)