Update CHANGELOG.md with S3 Vectors support for Knowledge Base

Author: Bob Strahan

CHANGELOG.md

@@ -5,6 +5,16 @@ SPDX-License-Identifier: MIT-0
 
 ## [Unreleased]
 
+## [0.3.16]
+
+### Added
+
+- **S3 Vectors Support for Cost-Optimized Knowledge Base Storage**
+  - Added S3 Vectors as alternative vector store option to OpenSearch Serverless for Bedrock Knowledge Base with 40-60% lower storage costs
+  - Conditional KMS policy implementation that only grants S3 Vectors indexing service permissions when `KnowledgeBaseVectorStore = "S3_VECTORS"`
+  - Custom resource Lambda implementation for S3 vector bucket and index management (using boto3 s3vectors client) with proper IAM permissions and resource cleanup
+  - Unified Knowledge Base interface supporting both vector store types with automatic resource provisioning based on user selection
+
 ### Fixed
 - Fixed issue where CloudFront policy statements were still appearing in generated GovCloud templates despite CloudFront resources being removed
 - Fix duplicate Glue tables are created when using a document class that contains a dash (-). Resolved by replacing dash in section types with underscore character when creating the table, to align with the table name generated later by the Glue crawler - resolves #57.