docs: add authentication section to readme.

Document the new and existing authentication methods.

Author: jmcarp

README.md

@@ -57,3 +57,45 @@ func main() {
 	fmt.Printf("%+v\n", resp)
 }
 ```
+
+### Authentication
+
+The client supports several authentication methods.
+
+1. Explicit configuration: Set `Host` and `Token` in the `Config`:
+
+   ```go
+   cfg := oxide.Config{
+       Host:  "https://api.oxide.computer",
+       Token: "oxide-abc123",
+   }
+   ```
+
+1. Environment variables: Set `OXIDE_HOST` and `OXIDE_TOKEN`:
+
+   ```bash
+   export OXIDE_HOST="https://api.oxide.computer"
+   export OXIDE_TOKEN="oxide-abc123"
+   ```
+
+1. Oxide profile: Use a profile from the Oxide config file:
+   - Set `Profile` in the `Config`:
+     ```go
+     cfg := oxide.Config{
+         Profile: "my-profile",
+     }
+     ```
+   - Or set the `OXIDE_PROFILE` environment variable:
+     ```bash
+     export OXIDE_PROFILE="my-profile"
+     ```
+
+1. Default profile: Use the default profile from the Oxide config file:
+   ```go
+   cfg := oxide.Config{
+       UseDefaultProfile: true,
+   }
+   ```
+
+When using profiles, the client reads from the Oxide CLI configuration files located at `$HOME/.config/oxide/credentials.toml` (or a custom directory via `Config.ConfigDir`).
+Values defined in `Config` have higher precedence and override environment variables. Configuring both profile and host/token authentication is disallowed and will return an error from oxide.NewClient, as well configuring both a profile and the default profile.