Updated API doc link in README
Updated API doc link in README

API reference link now points to docs hosted on https://ibm-bluemix-mobile-services.github.io
ibm-bluemix-mobile-services/bms-clientsdk-swift-analytics,ibm-bluemix-mobile-services/bms-clientsdk-swift-analytics,ibm-bluemix-mobile-services/bms-clientsdk-swift-analytics,ibm-bluemix-mobile-services/bms-clientsdk-swift-analytics,ibm-bluemix-mobile-services/bms-clientsdk-swift-analytics