Add documentation about association prefetching in queries

I found this neat feature in the code and Changelog, but it was nowhere to be found in the doc... So I documented it.

```js
// Sequelize v1.6.0 introduced prefetching of related objects for find() 
// and findAll(), using the new include query operator. 
//Here is how to fetch a phone numbers and its owner in one query:
PhoneNumber.find({ where: { id: 123 }, include: ['Person'] }).success(function(phoneNumber) {
  // now instead of using the getPerson() asynchronous accessor
  // you can access the `person` property directly, ans synchronously
  console.log(phoneNumber.person.firstName);
})
```