Update credential docs for gnome-secret.
Update credential docs for gnome-secret.

This can now be installed directly from crates.io, which is generally
much faster than git.

rustyhorde/cargo,rustyhorde/cargo,eminence/cargo,rust-lang/cargo,rustyhorde/cargo,rustyhorde/cargo,rust-lang/cargo,nipunn1313/cargo,eminence/cargo,nipunn1313/cargo,rust-lang/cargo,rust-lang/cargo,nipunn1313/cargo,eminence/cargo,nipunn1313/cargo,eminence/cargo