Updated the README.md to include swift
Updated the README.md to include swift

Including Swift in the metadata to be parsed by http://esri.github.io
benstoltz/arcgis-runtime-samples-ios,ShengQiangLiu/arcgis-runtime-samples-ios,sugar2010/arcgis-runtime-samples-ios,Natoto/arcgis-runtime-samples-ios,Esri/arcgis-runtime-samples-ios,benstoltz/arcgis-runtime-samples-ios,Esri/arcgis-runtime-samples-ios,Esri/arcgis-runtime-samples-ios,Esri/arcgis-runtime-samples-ios,ShengQiangLiu/arcgis-runtime-samples-ios,Natoto/arcgis-runtime-samples-ios,sugar2010/arcgis-runtime-samples-ios,rolson/arcgis-runtime-samples-ios