docs(app): clarify CSS and SVG icons usage
docs(app): clarify CSS and SVG icons usage

dballance/ng-lightning,esarbanis/ng-lightning,dballance/ng-lightning,ng-lightning/ng-lightning,dballance/ng-lightning,esarbanis/ng-lightning,ppolyzos/ng-lightning,ihym/ng-lightning,ppolyzos/ng-lightning,ng-lightning/ng-lightning,ihym/ng-lightning,ihym/ng-lightning,ppolyzos/ng-lightning,esarbanis/ng-lightning