update(readme): add plunkr+systemjs information
update(readme): add plunkr+systemjs information

Adds image and link to the online Plunkr template: https://plnkr.co/edit/h8hzyoEyqdCXmTBA7DfK?p=preview

brendanoffer/flex-layout,karlhaas/flex-layout,DevVersion/flex-layout,d3viant0ne/flex-layout,d3viant0ne/flex-layout,atscott/flex-layout,angular/flex-layout,karlhaas/flex-layout,benbraou/flex-layout,DevVersion/flex-layout,benbraou/flex-layout,angular/flex-layout,benbraou/flex-layout,atscott/flex-layout,brendanoffer/flex-layout,angular/flex-layout,DevVersion/flex-layout,d3viant0ne/flex-layout,atscott/flex-layout,benbraou/flex-layout,karlhaas/flex-layout,brendanoffer/flex-layout,atscott/flex-layout,angular/flex-layout