Small update to README
Small update to README

AdjustIo is a subdirectory, parameter dictionary should only contain strings.
mseegers/ios_sdk,trademob/ios_sdk,akolov/adjust,BlueCrystalLabs/ios_sdk,pitchtarget/adjust-ios-sdk,nicolas-brugneaux-sociomantic/ios_sdk,yutmr/ios_sdk,dannycxh/ios_sdk,Threadflip/adjust_ios_sdk