Add documentation for backend_config and backend_config_file inputs

Author: dflook

terraform-unlock-state/README.md

@@ -22,6 +22,31 @@ Force unlocks a Terraform remote state.
   - Optional
   - Default: `default`
 
+* `backend_config`
+
+  List of terraform backend config values, one per line.
+
+  ```yaml
+  with:
+    backend_config: token=${{ secrets.BACKEND_TOKEN }}
+  ```
+
+  - Type: string
+  - Optional
+
+* `backend_config_file`
+
+  List of terraform backend config files to use, one per line.
+  Paths should be relative to the GitHub Actions workspace
+
+  ```yaml
+  with:
+    backend_config_file: prod.backend.tfvars
+  ```
+
+  - Type: string
+  - Optional
+
 * `lock_id`
 
   The ID of the state lock to release