Updated README
Updated README

Added some notes reganding eventIds and callback parameters.

Threadflip/adjust_ios_sdk,nicolas-brugneaux-sociomantic/ios_sdk,yutmr/ios_sdk,mseegers/ios_sdk,trademob/ios_sdk,akolov/adjust,BlueCrystalLabs/ios_sdk,pitchtarget/adjust-ios-sdk,dannycxh/ios_sdk