avoid-leaking-state missing case example

https://github.com/ember-cli/eslint-plugin-ember/blob/master/docs/rules/avoid-leaking-state-in-ember-objects.md

Hey,
I see a problem with `good` example which doesn't replace the `bad` example functionality in 100%. If you would set properties in `init` hook you will lose a possibility to override it by passing it during creation.

A full example of the problem:

```js
let leakingState = EmberObject.extend({
items: []
});

leakingState.create({ items: [1,2,3] });
// I know the state is leaking but user expect to get a passed list
leakingState.get('items') // returns [1,2,3] OK!

let NoLeakingState = EmberObject.extend({
init() {
this._super(...arguments);
this.items = [];
},
});

NoLeakingState.create({ items: [1,2,3] });
NoLeakingState.get('items') // returns [] NOT OK!

// A suggested solution

let NoLeakingStateNew = EmberObject.extend({
items: computed(function() {
return [];
})
});

NoLeakingStateNew.create({ items: [1,2,3] });
NoLeakingStateNew.get('items') // returns [1,2,3] OK!
```

So, we should add a new example with explanation of the problem or replace it. I know, it depends on the use case but in my opinion, a much more common expected behavior is the one with the computed.