Add GITSIGN_TOKEN_PROVIDER docs (#447)

Missed from last PR.

Signed-off-by: Billy Lynch <[email protected]>

README.md

@@ -82,6 +82,7 @@ The following config options are supported:
 | ---------------------------- | ------------------ | -------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
 | GITSIGN_CREDENTIAL_CACHE     |                    |                                  | Optional path to [gitsign-credential-cache](cmd/gitsign-credential-cache/README.md) socket.                                                                                                                                                                                                                                                                                                                           |
 | GITSIGN_CONNECTOR_ID         | ✅                 |                                  | Optional Connector ID to auto-select to pre-select auth flow to use. For the public sigstore instance, valid values are:<br>- `https://github.com/login/oauth`<br>- `https://accounts.google.com`<br>- `https://login.microsoftonline.com`                                                                                                                                                                            |
+| GITSIGN_TOKEN_PROVIDER       | ✅                 |                                  | Optional OIDC token provider to use to fetch tokens. If not set, any available providers are used. valid values are:<br>- `interactive`<br>- `spiffe`<br>- `google-workload-identity`<br>- `google-impersonation`<br>- `github-actions`<br>- `filesystem`<br>- `buildkite-agent`                                                                                                                                      |
 | GITSIGN_FULCIO_URL           | ✅                 | https://fulcio.sigstore.dev      | Address of Fulcio server                                                                                                                                                                                                                                                                                                                                                                                              |
 | GITSIGN_LOG                  | ❌                 |                                  | Path to log status output. Helpful for debugging when no TTY is available in the environment.                                                                                                                                                                                                                                                                                                                         |
 | GITSIGN_OIDC_CLIENT_ID       | ✅                 | sigstore                         | OIDC client ID for application                                                                                                                                                                                                                                                                                                                                                                                        |