message: Fix docs generator howto
  * Fix docs generator howto
  We need to run `setup.py` to recompile the c++ files before regenerating the docs.
  * Update CONTRIBUTING.md
diff: @@ -30,7 +30,7 @@ Then, after you have made changes to Python and C++ files:
  ## Generated operator documentation
  -[Operator docs in Operators.md](Operators.md) are automatically generated based on C++ operator definitions. To refresh these docs, run the following command from the repo root and commit the results:
  +[Operator docs in Operators.md](Operators.md) are automatically generated based on C++ operator definitions. To refresh these docs, remember to re-install (see above) and then run the following command from the repo root and commit the results:
  ```
  python onnx/defs/gen_doc.py
  