Document Rc<T>

```
{
    let five_alive = {
        // 5 is allocated  
        let five = Rc::new(5i);
        // points to the same 5
        let also_five = five.clone(5i);
        // also points to the same 5
        also_five.clone()
        // five and also_five go out of scope here, but a clone lives on in five_alive
        // so the 5 isn't deallocated
    };
    // five_alive goes out of scope, so there are no Strong references to 5, and it is destructed.
    // There are also no weak references to 5, so its memory is also deallocated.
}
```