I.e. https://github.com/flutter/engine/blob/main/tools/engine_tool/README.md

We should explain:

• The intention of the tool (what it does and does not do)
• The golden path (building a target configuration)
• Using it to run tests (C++, experimental support for Dart)
• Using it to run a Flutter app with a local engine build
• Passing custom GN args (i.e. for a prebuilt Dart SDK, perhaps others)
• Optimizing builds with RBE (googlers only)