π₁(S¹) ≅ ℤ lacks motivation

[simnalamburt]

Although Napkin spends considerable time preparing to prove $\pi_1(S^1) \cong \mathbb{Z}$, it doesn’t present any applications of this result. How about borrowing these two examples from [Ha02] and incorporating them somewhere in the book?

[vEnhance]

I'd be happy with a PR for an optional section that shows one or both of these examples. (I think optional because I think $\pi_1(S^1) \cong \mathbb Z$ seems interesting / obviously important in its own right, and the two examples here aren't reused later on, so it feels like a side-quest section would be better than a mainline one.)

[vEnhance]

Else, I'll add a short remark myself later this summer when I have a chance to breathe.

[simnalamburt]

I’ll go ahead and create a PR as you suggested.