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.
Same as above, except `[:]` is even uglier. For both cases, I'd probably reconsider in the case of properties since explicit typing is more common, but I'd have very hard time doing that for every array or dictionary I declare elsewhere.