Point out resource cleanup in vulkano.rs guide

### Feature description So I just spent **a lot** of time trying to figure out why my application was leaking memory, just to figure out that `cleanup_finished` is an important function that has to be manually called. I started with the vulkano.rs guide to get familiar with Vulkan and Vulkano, but that unfortunately doesn't seem to mention this cleanup function anywhere. The examples do use this and especially the `triangle` example does a nice job of pointing out the importance of the function, but since I had large parts of my Vulkan code already in place after reading the vulkano.rs guide, I apparently didn't pay enough attention when I filled in the blanks using the examples. Maybe this is just me being dumb, but on the off chance I'm not the first, it might be worthwhile to think about making sure I'll be the last.