Update README.md

I am using this library with cocoapod, but have same issue with:
 - https://github.com/iziz/libPhoneNumber-iOS/issues/11
 - plist file exists in the bundle path, but exception occurs while unarchiveObject in my emulator. 

My solution is: 
 1. update CocoaPod to the latest
 2. fix Podfile and install again.