message: Add recursive flag to the submodule command
diff: @@ -89,7 +89,7 @@ You can also build and install ONNX locally from source code:
  ```
  git clone https://github.com/onnx/onnx.git
  cd onnx
  -git submodule update --init
  +git submodule update --init --recursive
  python setup.py install
  ```
  