cargo package should warn binary files by default

[upsuper]

I've seen multiple times that some images which is used in README only is included into crate package, e.g. bindgen contains a 1.3MB PNG, and dtoa contains a 68KB PNG.

I believe binary files in general are not useful in packages. They should be excluded or at least warned by default. People should still be able to explicitly specify binary files if necessary, though.