Idea: emphasize ability to check multiple targets in documentation and help message

[vi]

Like usual Cargo invocation, cargo hack can check multiple targets simultaneously if you specify multiple --target options.

This fact is however not very well known and is easy to miss; users may reach for cargo-hack to iterate targets, just like cargo-hack iterates features.

I suggest to add a reminder about this in README, also maybe mention --target explicitly in the CLI help message.

[taiki-e]

I would accept PR that does this.