Show ES6 alternative syntax for importing

[Cplantijn]

Some users may be new to using ES6. This change may provide a hint to those who are new to ES6 but would like to maintain as much ES6 syntax as possible.

[tricoder42]

If you import polyfill using ES6 syntax, you must call imported function:

// require('es6-promise').polyfill();  // polyfill is called

import { polyfill } from 's6-promise';
polyfill();

[louy]

polyfill() does nothing. should be import 'es6-promise';
stefanpenner/es6-promise@8727651#diff-1f5f0efd56c19eac9e37ef47e6ca89fcL13