message: Add debug instruction
diff: @@ -7,6 +7,11 @@ To run the Outline Manager:
  yarn do server_manager/electron_app/run
  ```
  +## Debug an existing binary
  +
  +You can run an existing binary in debug mode by setting `OUTLINE_DEBUG=true`.
  +This will enable the Developer menu on the application window.
  +
  ## Packaging
  To build the app binary:
  