adds notice to index

Signed-off-by: Kyle Davis <[email protected]>

index.md

@@ -121,3 +121,8 @@ See [opensearch.org/javadocs/](https://opensearch.org/javadocs/).
 [OpenSearch](https://opensearch.org) is supported by Amazon Web Services. All components are available under the [Apache License, Version 2.0](https://www.apache.org/licenses/LICENSE-2.0.html) on [GitHub](https://github.com/opensearch-project/).
 
 The project welcomes GitHub issues, bug fixes, features, plugins, documentation---anything at all. To get involved, see [Contributing](https://opensearch.org/source.html) on the OpenSearch website.
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+<small>OpenSearch includes certain Apache-licensed Elasticsearch code from Elasticsearch B.V. and other source code. Elasticsearch B.V. is not the source of that other source code. ELASTICSEARCH is a registered trademark of Elasticsearch B.V.</small>