cargo-credential-1password: Add configuration instructions

Author: Turbo87

credential/cargo-credential-1password/README.md

@@ -2,10 +2,25 @@
 
 A Cargo [credential provider] for [1password].
 
+## Usage
+
 `cargo-credential-1password` uses the 1password `op` CLI to store the token. You
 must install the `op` CLI from the [1password
 website](https://1password.com/downloads/command-line/).
 
+Afterward you need to configure `cargo` to use `cargo-credential-1password` as
+the credential provider. You can do this by adding something like the following
+to your [cargo config file][credential provider]:
+
+```toml
+[registry]
+global-credential-providers = ["cargo-credential-1password --account my.1password.com"]
+```
+
+Finally, run `cargo login` to save your registry token in 1password.
+
+## CLI Arguments
+
 `cargo-credential-1password` supports the following command-line arguments:
 
 * `--account`: The account name to use. For a list of available accounts,