message: Update README.md for non-conda build from source
diff: @@ -61,6 +61,13 @@ You can then install ONNX from PyPi:
  pip install onnx
  ```
  +Note: When installing in a non-Anaconda environment, make sure to install the Protobuf compiler before running the pip installation of onnx. For example, on Ubuntu:
  +
  +```
  +sudo apt-get install protobuf-compiler libprotoc-dev
  +pip install onnx
  +```
  +
  After installation, run
  ```
  