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.
Why don't you just add final line to the function before the return value that does something like `y = x + 4` and then  `return y` so that you can explicitly show that the compiler is going to have to do something smart for y to handle both type cases for `x`? (I'm not sure if this is going to ruin the discussion that follows.)