Add guideline for creating mutable collections

The alternative syntax is:

```swift
var collection = [Object]()
```

I find this syntax jarring because of the visual combination of the braces and
the parens. It also seems to be taking a too-rigid stance in favor of using
type inference. I believe that the proposed syntax lets you type the object,
and then be explicit about creating an empty array, which will then by typed
appropriately.
What's the objective reason behind this choice? 

To me `var collection = [Object]()` is:
- Obvious: it can't be anything else but an array of `Object` instances. It also adheres more to the 'Use type inference where the end result is obvious' cited above.
- Looks nicer: no extra `:` or duplicate `[ ]`. The additional `()` are standard for initialization.
- Readable
- Used by Apple (See "Creating and Initializing an Array" under "Collection types")

I'd be fine using the suggested syntax as long as there is a good reason. So far I don't see any.