docs: add v0.5.2 release notes

Add changelog entries for recent commits:
- Health package: Kubernetes health probes
- Logger package: runtime level control and fixes
- Database package: config file override support

CHANGELOG.md

@@ -1,3 +1,22 @@
+# Release Notes - v0.5.2
+
+## Health Package
+- **Kubernetes-native health probes** - Added dedicated handlers for liveness (`/healthz`), readiness (`/readyz`), and startup (`/startupz`) probes
+- **Flexible configuration options** - New `WithLivenessHandler`, `WithReadinessHandler`, `WithStartupHandler`, and `WithServiceName` options
+- **JSON response formats** - Structured probe responses with service identification
+- **Backward compatibility** - Maintains existing `/__health` and `/` endpoints
+
+## Logger Package
+- **Runtime log level control** - Independent level management for stderr and OTLP loggers via `SetLevel()` and `SetOTLPLevel()`
+- **Environment variable support** - Configure levels with `LOG_LEVEL` and `OTLP_LOG_LEVEL` env vars
+- **String parsing utility** - New `ParseLevel()` function for converting string levels to `slog.Level`
+- **Buffering exporter fix** - Resolved "unlock of unlocked mutex" panic in `bufferingExporter`
+- **Initialization redesign** - Eliminated race conditions in TLS/tracing setup retry logic
+
+## Database Package
+- **Configuration file override** - Added `DATABASE_CONFIG_FILE` environment variable to specify custom database configuration file paths
+- **Flexible path configuration** - Override default `["database.yaml", "/vault/secrets/database.yaml"]` search paths when needed
+
 # Release Notes - v0.5.1
 
 ## Observability Enhancements