Add a small section on notation / explanation of conditional probability?

[WillTirone]

Love the book so far :) I noticed that there wasn't any mention of the definition of conditional probability, i.e. "expectation of Y given T = 1" or the definition of an expectation. I'm sure you're assuming some basic stats knowledge, but since there's a section on stats review, I thought these might be beneficial.

I'm happy to type up a few short paragraphs or propose some changes, let me know if that's something that you would like!