[racket] Teach Yourself Racket (version 0.1)

I've been meaning for some time to write something called Teach Yourself 
Racket, and finally scraped an alpha version together under threat of 
classes starting (I'm going to use it in my grad course). I'd appreciate 
comments to help me improve it.

https://cs.uwaterloo.ca/~plragde/tyr/

Followups by private e-mail to me, please, unless they're really 
relevant to the community at large. Thanks. --PR