Prepare CHANGELOG.md for v1.5.0 release (#349)

Signed-off-by: Thomas Farr <tsfarr@amazon.com>

CHANGELOG.md

@@ -2,6 +2,8 @@
 Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 
 ## [Unreleased]
+
+## [1.5.0]
 ### Fixed
 - Fix highlight max_analyzer_offset field name to match with the one introduced in OpenSearch 2.2.0 ([#322](https://github.com/opensearch-project/opensearch-net/pull/322))
 - Fix null-ref exception when track total hits is disabled ([#341](https://github.com/opensearch-project/opensearch-net/pull/341))
@@ -85,6 +87,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 - Bumps `System.Diagnostics.DiagnosticSource` from 5.0.0 to 6.0.0
 - Bumps `Microsoft.NETFramework.ReferenceAssemblies` from 1.0.0-preview.2 to 1.0.3
 
-[Unreleased]: https://github.com/opensearch-project/opensearch-net/compare/v1.4.0...1.x
+[Unreleased]: https://github.com/opensearch-project/opensearch-net/compare/v1.5.0...1.x
+[1.5.0]: https://github.com/opensearch-project/opensearch-net/compare/v1.4.0...v1.5.0
 [1.4.0]: https://github.com/opensearch-project/opensearch-net/compare/v1.3.0...v1.4.0
 [1.3.0]: https://github.com/opensearch-project/opensearch-net/compare/v1.2.0...v1.3.0