message: Add mypy to CONTRIBUTING
diff: @@ -52,6 +52,29 @@ pytest
  to begin the tests.
  +# Static typing (mypy)
  +
  +We use [mypy](http://mypy-lang.org/) to run static type checks on the onnx code base. To check that your code passes, you'll first need to install the mypy type checker. If you're using python 3, call from your onnx source folder:
  +
  +```
  +pip install -e .[mypy]
  +```
  +
  +The type checker cannot run in a python 2 environment (but it will check python 2 code).
  +If you're using python 2, you need to install mypy into your system packages instead:
  +
  +```
  +pip3 install mypy==[version]
  +```
  +*Note: You'll find the version we're currently using in `setup.py`.*
  +
  +After having installed mypy, you can run the type checks:
  +
  +```
  +python setup.py typecheck
  +```
  +
  +
  # Other developer documentation
  * [How to implement ONNX backend (ONNX to something converter)](ImplementingAnOnnxBackend.md)
  