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.
Ah, so this happens even if the type is inferred. (I suspected, but wasn't sure until we got here.) This might make it better to lightly explain how a variable is initially assigned a type in Julia -- either by explicit type annotation, or by basic type inference by the compiler. This could be a short, relevant fact explained in this section, or even in your Julia tutorial I suppose.