Update README for distribution level BWC tests (#3645)

Signed-off-by: Zelin Hao <[email protected]>

src/test_workflow/README.md

@@ -110,6 +110,17 @@ To run OpenSearch Dashboards BWC tests.
 opensearch-dashboards=https://ci.opensearch.org/ci/dbc/distribution-build-opensearch-dashboards/1.3.5/4056/linux/x64/tar
 ```
 
+#### BWC tests on distribution bundle level
+
+The BWC tests running on distribution level are using the same framework from OpenSearch. The test cluster is spin up with the `latest` distribution bundle of provided version exclusively when the project is initialized with property `-PcustomDistributionDownloadType=bundle`. In this repo, the test workflow will be enable this gradle property by default.[BWC test script](https://github.com/opensearch-project/opensearch-build/blob/edffab782abf96390b3993ccc92425c63ab77884/scripts/default/bwctest.sh#L38)
+
+Example distribution bundle URL: `https://ci.opensearch.org/ci/dbc/distribution-build-opensearch/1.3.2/latest/linux/x64/tar/dist/opensearch/opensearch-1.3.2-linux-x64.tar.gz`
+This feature for BWC testing is supported for distribution versions starting `v1.3.2`.
+
+On CI level for plugins, security certificates need to be manually imported when spinning up the test cluster as security plugin is included in the distribution bundle. When upgrading the version within the test cluster, `nextNodeToNextVersion` is used for a single node upgrade and `goToNextVersion` is for a full restart upgrade.
+
+See [anomaly-detection#766](https://github.com/opensearch-project/anomaly-detection/pull/766) or [observability#1366](https://github.com/opensearch-project/observability/pull/1366) for more information.
+
 ### Performance Tests
 
 TODO: Add instructions for running performance tests with `test.sh`