One of the most common questions I get when working with iOS developers is How do I add the Salesforce Mobile SDK for iOS to my existing project? The good news is that it is much easier than you think. I just wrote a tutorial on how to do it.
The tutorial covers the latest version of the Mobile SDK, and uses Swift 2 for the code examples, including a fully Swift-ified AppDelegate. (come on, admit it, we don’t want to be writing Objective-C any more, do we?) The tutorial also includes an example of how you can design your app so the Salesforce login feels more part of your app flow, rather than being the very first thing that pops up when a user starts their app.
Give the tutorial a go, and let me know if it helps.