Add documentation on debugging by generating a `.gpuTrace` file

[chinedufn]

https://github.com/gfx-rs/metal-rs/pull/151#issuecomment-653904768

This approach seems like less of a hassle since you don't need to wire up build configurations in Xcode.

Xcode becomes nothing more than a place to view your .gpuTrace file.

[chinedufn]

Although I suppose since this is just code it should be incorporated into an example.

Perhaps we can edit the window example to capture the first few frames to a .gpuTrace file.

Then have the debugging docs simply introduce that this is possible and then link to the window example in order to see how it's done.

[kvark]

That sounds great!

[chinedufn]

Note to self - Xcode is required in order to generate a .gpuTrace https://github.com/gfx-rs/metal-rs/pull/155#discussion_r451534460

So the docs can demonstrate how ti generate a trace file using the least amount of Xcode-ing possible.