added a more detailed theme generator section in Readme
added a more detailed theme generator section in Readme

tomwayson/generator-esri-appbuilder-js,tomwayson/generator-esri-appbuilder-js,Esri/generator-esri-appbuilder-js,g3r4n/generator-wab,purnachand227/generator-esri-appbuilder-js,purnachand227/generator-esri-appbuilder-js,Esri/generator-esri-appbuilder-js,g3r4n/generator-wab,Esri/generator-esri-appbuilder-js