The new chapter is on kinds of arrows. I have also revised Chapter 3 (now preferring to talk about implementing structures in set theory, rather than to talk of set-theoretic surrogates or proxies — the change of rhetoric isn’t really a change of view, but will I hope slightly mollify some readers!).
I should say that Beginning Category Theory is very much a work in progress, and I can imagine these early chapters getting significantly revised to better fit the later ones in content and tone. But, for all that, I’m putting them online as I go along, when I get to the point of thinking that a new chapter is at least better than the corresponding old one in the Gentle Intro!