CHANGELOG.md: update

Author: razzmatazz

CHANGELOG.md

@@ -13,6 +13,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
   - https://github.com/razzmatazz/csharp-language-server/pull/228
 * Implement (experimental) support for loading multi-tfm solutions
   - https://github.com/razzmatazz/csharp-language-server/pull/205
+* Implement workspace diagnostics
+  - https://github.com/razzmatazz/csharp-language-server/pull/235
 
 ## [0.17.0] - 2025-04-30 / Krokšlys
 * Upgrade Roslyn to 4.13.0