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.
Good. Once I got to this part of the explanation, I understood exactly what you meant. One the explanation in the paragraph https://github.com/aosabook/500lines/pull/65/files#diff-e3506aeeec9f29ecbcd9311234265543R169 is cleaned up a bit, this will be a really well-paced explanation of the problem at hand.