Document that cargo binstall can be used to install cargo-outdated

[alilleybrinker]

cargo binstall cargo-outdated works, but you have to know that cargo binstall exists and try it out to discover this. It's a much easier experience using the pre-built binaries if they're available, and it would be nice if cargo-outdated's install instructions in the README documented that you can use cargo binstall if you want!