Added example of promise constructor

[G-Rath]

Addresses #99

I opted not to use ES6 shorthand here, as I think it's nicer to make clear that the property name is 'Promise'.

If it's not clear enough, I'm happy to also add an example of using Bluebird or another Promise library.

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[G-Rath]

I signed it!

[googlebot]

CLAs look good, thanks!

[stephenmcd]

That's great, thanks a lot!

[0xpatrickdev]

Hey @G-Rath, this is great.

@stephenmcd I'm not sure how you manage the source/build for the gh-pages site, but it would be helpful to have this example on there too. I can create a PR on the gh-pages branch if that's where you manage it.

[stephenmcd]

@pcooney10 I've just updated the docs site