Clarify how to run with CocoaPods, change formatting in part 2

After working at BattleHack, we had quite a few questions about how to actually start using CocoaPods, which led to this.

Plus I just wanted to make section 2's formatting prettier, and section 3's section more active-voice.

Review: @ayanonagon @diogeneshamilton 
Maybe add something to make it clear that you run `pod init` only if you don't already have a `Podfile` :+1: