cli: rename 'list' command to 'readers'; improve usage

There are going to be several `list` commands (e.g. `yubikey keys list`)
so this is a confusing name.

If we need more than one `readers` subcommand we can change this to be
`readers list` eventually.

Separately (in what probably should've been its own commit, mea culpa)
this adds slightly better usage.

cli/Cargo.toml

@@ -2,10 +2,9 @@
 name = "yubikey-cli"
 version = "0.0.1"
 description = """
-Command-line interface for performing encryption and signing using RSA and/or
-ECC keys stored on YubiKey devices.
+Command-line interface for performing encryption and signing using RSA/ECC keys stored on YubiKey devices.
 """
-authors = ["Tony Arcieri <bascule@gmail.com>"]
+authors = ["Tony Arcieri <tony@iqlusion.io>"]
 edition = "2018"
 license = "BSD-2-Clause"
 repository = "https://github.com/iqlusioninc/yubikey-piv.rs"