Add ES2015+ example

[jamonholmgren]

I had the same question as @abimelex in #275, and figured it would be useful to have a short example in the README.

[mislav]

I don't think that the instructions for loading this polyfill are different from any other polyfill out there (no exports). Therefore I don't find this particularly useful. But if people are constantly looking this up, then I guess it can't hurt to clear this up. Thoughts @dgraham?

[jamonholmgren]

You're right, it's no different than other polyfills...this was the first polyfill I've used within our new Babel environment, so it was the first time I'd considered this question.

[dgraham]

Several people have found the answer in #275 useful, so promoting it to the README might help people importing modules for side effects, like adding window global polyfills.

[jamonholmgren]

Thank you!