Add basic example to readme
Add basic example to readme

Looking at this, I'm thinking that I might want to change the API a bit.
Mostly by having addEventListener return an object that has everything
that removeEventListener needs to remove the eventListener. So, it would
instead look like this:

```js
const listenerHandle = addEventListener(
  window,
  'scroll',
  () => { console.log('scrolling') },
  { passive: true }
);

...

removeEventListener(listenerHandle);
```

lencioni/consolidated-events