adding npm installation note to readme/web guide
adding npm installation note to readme/web guide

BranchMetrics/Smart-App-Banner-Deep-Linking-Web-SDK,BranchMetrics/Smart-App-Banner-Deep-Linking-Web-SDK,BranchMetrics/Smart-App-Banner-Deep-Linking-Web-SDK,BranchMetrics/Smart-App-Banner-Deep-Linking-Web-SDK