feat: add tar_path documentation (#184)

DOCS.md

@@ -268,6 +268,7 @@ The following parameters are used to configure the image:
 | `snapshot_mode`        | control how to snapshot the filesystem. - options: `full`, `redo`, or `time`                                            | `false`  | `N/A`             | `PARAMETER_SNAPSHOT_MODE`<br>`KANIKO_SNAPSHOT_MODE`               |
 | `tag`                  | tag generated for build                                                                                                 | `false`  | **set by Vela**   | `PARAMETER_TAG`<br>`KANIKO_TAG`<br>`VELA_BUILD_TAG`               |
 | `tags`                 | unique tags of the image                                                                                                | `true`   | `latest`          | `PARAMETER_TAGS`<br>`KANIKO_TAGS`                                 |
+| `tar_path`        | save the image as a tarball at path                                                                                  | `false`  | `N/A`           | `PARAMETER_TAR_PATH`<br>`KANIKO_TAR_PATH`               |
 | `target`               | set the target build stage for the image                                                                                | `false`  | `N/A`             | `PARAMETER_TARGET`<br>`KANIKO_TARGET`                             |
 | `username`             | user name for communication with the registry                                                                           | `true`   | `N/A`             | `PARAMETER_USERNAME`<br>`KANIKO_USERNAME`<br>`DOCKER_USERNAME`    |
 | `custom_platform`      | set the custom platform for the image                                                                                   | `false`  | `N/A`             | `PARAMETER_CUSTOM_PLATFORM`<br>`KANIKO_CUSTOM_PLATFORM`           |