Adding instructions for adding the SDK to existing apps

Here's a [preview](https://github.com/khawkins/SalesforceMobileSDK-iOS-Distribution/blob/readme_update/README.md) of the new README.md.

NB: I didn't create instructions for adding the Mobile SDK to an existing hybrid app, because the it would be such an extensive set of changes, compared to just generating a hybrid app from the template, that it seems prohibitive to even take that approach.