Warn about no useful metadata in `cargo package`.

A lack of description or any URL in a crate mean that crates.io can't
display *any* information about what it does other than the name, and
the only way for a human to work this out is to download the package as
a dependency locally, or to manually track down the repo/documentation.

This is rather unfriendly for people looking for packages, and the way
to fix it (adding some keys in Cargo.toml) is not immediately obvious.
Hence `cargo` can be a little intelligent and provide some guidance.

Closes #902.