add import information for installing onnxmltools from source code (#19)

* add import information for installing onnxmltools from source code

* Simplify the description

Author: FrozenGene

README.md

@@ -25,6 +25,7 @@ or install from source:
 ```
 pip install git+https://github.com/onnx/onnxmltools
 ```
+Note:If you choose to install `onnxmltools` from its source code, you must set an environment variable `ONNX_ML=1` before installing `onnx` package.
 
 ## Dependencies
 This package uses ONNX, NumPy, and ProtoBuf. If you are converting a model from scikit-learn or Apple Core ML you need the following packages installed respectively: