Add Opensearch driver to README.md

[dhojayev]

Hi,
Let me first thank you for the great package that we use daily in our operations.

Our team has developed a driver to enable applying migration to AWS Opensearch. Maybe it is worth mentioning it in README.md to help others to find it easier.

https://github.com/limangotech/opensearch-driver

Please let me know if you need any other info from my side.

Thanks.