Add onboarding doc for smoke tests

ONBOARDING.md

@@ -35,7 +35,7 @@ Add the new plugin to the [opensearch-plugins meta](https://github.com/opensearc
 
 ### Onboard to Test Workflow
 
-1. Update the test configuration file (use 1.3.0 as an example), [opensearch-1.3.0-test.yml](https://github.com/opensearch-project/opensearch-build/blob/opensearch-1.3.0/manifests/1.3.0/opensearch-1.3.0-test.yml), for a particular release, to include your plugin. This test configuration defines full suite of tests - `integ`, `bwc`, that can be run on the plugin.
+1. Update the test configuration file (use 1.3.0 as an example), [opensearch-1.3.0-test.yml](https://github.com/opensearch-project/opensearch-build/blob/opensearch-1.3.0/manifests/1.3.0/opensearch-1.3.0-test.yml), for a particular release, to include your plugin. This test configuration defines full suite of tests - `integ`, `bwc`, `smoke` that can be run on the plugin.
 
 2. For integration testing, the `test-workflow` runs integration tests available in the plugin repository. You will need to add `integ-test` config for your plugin in opensearch-1.3.0-test.yml, [example](https://github.com/opensearch-project/opensearch-build/blob/opensearch-1.3.0/manifests/1.3.0/opensearch-1.3.0-test.yml).
 
@@ -47,6 +47,13 @@ Add the new plugin to the [opensearch-plugins meta](https://github.com/opensearc
 
     1. It supports two test configs - `with-security` and `without-security`, which runs test with security plugin enabled and disabled respectively. Choose one or both depending on what your plugin integration tests support.
 
+4. For smoke testing, The test-workflow runs smoke tests defined in the [smoke_tests_spec](src/test_workflow/smoke_test/smoke_tests_spec) directory for each major OpenSearch version. The smoke tests are executed based on the API specification provided in the [OpenSearch API Specification](https://github.com/opensearch-project/opensearch-api-specification) repository.
+
+    1. The smoke test framework currently supports testing the OpenSearch distribution and its plugins. It ensures that core OpenSearch functionalities and plugin-specific APIs behave as expected before release.
+
+    2. You will need to add a distinct YAML file for your plugin in the appropriate major version directory under [smoke_tests_spec](src/test_workflow/smoke_test/smoke_tests_spec) and reference the file name in the test manifest for the corresponding component.
+
+    3. In the YAML file, specify the API path, HTTP method, request body as parameter and heading if `Content-Type` differs from the default `application/json`. Refer to an example test definition [here](src/test_workflow/smoke_test/smoke_tests_spec/2.x/opensearch.yml) for guidance. 
 
 ## Standalone Component Onboarding