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 sort of at the edges of the feedback that you asked for, but the first thing I noticed about your first example was that we jumped in very quickly from the introduction. 

I think it would help to have 1-2 sentences either right before or right after your Julia primer about 
- what type checks we will be designing and implementing and 
- why you chose those checks (e.g. are they very useful to have? Or was it because they demonstrate interesting problems? etc.)