Document Dart symbol mapping file feature in changelog (#364)

Added details about uploading Dart symbol mapping file for Flutter issue symbolication. Includes instructions for generating and configuring the mapping file.

Author: buenaflor

CHANGELOG.md

@@ -1,5 +1,17 @@
 # Changelog
 
+## Unreleased
+
+### Features
+
+- Upload Dart symbol mapping file ([#347](https://github.com/getsentry/sentry-dart-plugin/pull/347))
+    - Enables symbolication of Flutter issue titles for obfuscated builds.
+    - Supported: Android and iOS
+    - Not supported (yet): macOS, Linux and Windows.
+    - Generate the mapping file: Add `--extra-gen-snapshot-options=--save-obfuscation-map=<path>` when building. Example: `flutter build apk --obfuscate --split-debug-info=build/symbols --extra-gen-snapshot-options=--save-obfuscation-map=build/mapping.json`
+    - Configure the plugin: Set `dart_symbol_map_path: build/mapping.json`
+    - Important: `dart_symbol_map_path` must point directly to the mapping file (absolute or relative path), not a directory.
+
 ## 3.2.0-beta.1
 
 ### Features