Update README for persistence
Update README for persistence

taptaptap/countly-sdk-ios,Countly/countly-sdk-ios,adan016/countly-sdk-ios,erkanyildiz/countly-sdk-ios,Navirize/countly-sdk-ios,erkanyildiz/countly-sdk-ios,Countly/countly-sdk-ios,Countly/countly-sdk-ios,IntergalacticInteractive/countly-sdk-ios,erkanyildiz/countly-sdk-ios,djz-code/countly-sdk-ios