Update CONTRIBUTING.md (flutter#6253)

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CONTRIBUTING.md

@@ -115,6 +115,7 @@ export LOCAL_DART_SDK=/path/to/dart-sdk
 dart ./tool/build_e2e.dart
 ```
 * Note: the LOCAL_DART_SDK needs to point to a local checkout of [dart-sdk](https://github.com/dart-lang/sdk/tree/main)
+* Note: if you are also developing server side code, e.g. the devtools_shared package, add a devtools_shared path override to `<path-to-dart-sdk>/pkg/dds/pubspec.yaml`.
 
 That will:
 - start the devtools server