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.
This is a bit confusing -- is it the fact that this function can return values of different types that causes type instability, or is it the assignment variable-type values to a previously-type-annotated variable? Your third sentence confirms that it's the latter, so I would just cut your second sentence out entirely here.