Add 2.18.0 release notes

[reta]

Description

Add 2.18.0 release notes

Related Issues

Part of https://github.com/opensearch-project/custom-codecs/issues/190

Check List

• [ ] New functionality includes testing.
• [ ] New functionality has been documented.
• [ ] API changes companion pull request created.
• [ ] Commits are signed per the DCO using --signoff.
• [ ] Public documentation issue/PR created.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license. For more information on following Developer Certificate of Origin and signing off your commits, please check here.