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.
Reda if u check my comment on the PR there is a good reason. I think that the way proposed is also very obvious and readable and looks nicer. () is typically used to init objects where as types don't use those in most languages so they look really strange there. Also I would argue that using something because apple uses it that way is a horrible reason. We all know they aren't known for the greatest code in their examples.