Document the SPM migration playbook for end users

Once RNSentry publishes a `Package.swift` and the React Native ecosystem supports SPM autolinking, end users will need clear guidance on:

* Whether to use CocoaPods or SPM for a given app.
* How to migrate an existing CocoaPods-based app to SPM.
* What the support matrix looks like (which RN versions support each path).
* What to do with Expo apps, brownfield iOS apps, and edge cases.

Without explicit guidance, users will hit the transition in an ad hoc way and surface confused support tickets to us.