docs: highlight vault using id token

Signed-off-by: Clement Bois <clement.bois@orange.com>

README.md

@@ -90,10 +90,12 @@ In order to be able to communicate with the Vault server, the variant requires t
 | Input / Variable | Description                            | Default value     |
 | ----------------- | -------------------------------------- | ----------------- |
 | `TBC_VAULT_IMAGE` | The [Vault Secrets Provider](https://gitlab.com/to-be-continuous/tools/vault-secrets-provider) image to use (can be overridden) | `registry.gitlab.com/to-be-continuous/tools/vault-secrets-provider:latest` |
-| `vault-base-url` / `VAULT_BASE_URL` | The Vault server base API url          | _none_ |
+| `vault-base-url` / `VAULT_BASE_URL` | The Vault server base API url          | **must be defined** |
 | `vault-oidc-aud` / `VAULT_OIDC_AUD` | The `aud` claim for the JWT | `$CI_SERVER_URL` |
-| :lock: `VAULT_ROLE_ID`   | The [AppRole](https://www.vaultproject.io/docs/auth/approle) RoleID | **must be defined** |
-| :lock: `VAULT_SECRET_ID` | The [AppRole](https://www.vaultproject.io/docs/auth/approle) SecretID | **must be defined** |
+| :lock: `VAULT_ROLE_ID`   | The [AppRole](https://www.vaultproject.io/docs/auth/approle) RoleID | _none_ |
+| :lock: `VAULT_SECRET_ID` | The [AppRole](https://www.vaultproject.io/docs/auth/approle) SecretID | _none_ |
+
+By default, the variant will authentifacte using a [JWT ID token](https://docs.gitlab.com/ee/ci/secrets/id_token_authentication.html). To use [AppRole](https://www.vaultproject.io/docs/auth/approle) instead the `VAULT_ROLE_ID` and `VAULT_SECRET_ID` should be defined as secret project variables.
 
 #### Usage
 
@@ -126,5 +128,4 @@ include:
 variables:
   # Secrets managed by Vault
   SONAR_TOKEN: "@url@http://vault-secrets-provider/api/secrets/b7ecb6ebabc231/my-app/sonar?field=token"
-  # $VAULT_ROLE_ID and $VAULT_SECRET_ID defined as a secret CI/CD variable
 ```