Document how to use in Browserify…

… as per #21. This is just a quick first pass, I'm almost certainly using too many words here.

- I've proposed changing the suggest ES6 Promise implementation to [`native-promise-only`](https://github.com/getify/native-promise-only) as I believe its API is more similar to the CommonJS API fetch has*.  Would you like me to change that back?
- Would you like me to squash my commits?
- Is ‘fuzzy versions’ clear enough or should I change this to ‘version ranges’?

\*  The equivalent for `es6-promise` is:
```js
require('es6-promise').polyfill(); // require('native-promise-only');
```

It would be nice if we can do #9 because it would simplify some of this explanation (around fuzzy versions/version ranges) greatly.
Ah, so `native-promise-only` doesn't require an explicit export when used in CJS mode.

I'd be cool recommending either. I guess thats the nice thing about it being optional is that you can use whatever promise polyfill you like.