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.
If I'm the only one arguing for this, then I'd obviously feel bad for standing in the way. If everyone else's tastes align, then I'll suck it up. But that won't change how I feel about this nor how I'd write code for my own projects or other open source projects I start.