ONNX README (#1656)

* add bot

* test

* Add mxnet operator reminder

* file changed by mistake

* file changed by mistake

* file changed by mistake

* add version

scripts/onnx/README.md

@@ -34,7 +34,14 @@ Inference with ONNX model is straightforward:
 
 ### Export GluonCV Models to ONNX
 
-For those who are interested, there are three main steps to export the model:
+For those who are interested, below is a guide on how to export the model yourself:
+
+To be notice, current released version of mxnet(1.8.0) does not support many operators. Therefore, in order to export models to onnx, you'll have to either
+
+1. Build the newest version of mxnet from source
+2. Use this [tool](https://github.com/apache/incubator-mxnet/blob/d10b76937b891964381d25ecea60e443a9876df1/tools/onnx/update_onnx.py) to add newly supported operators to your mxnet
+
+There are three main steps to export the model:
 
 1. Get the GluonCV model using `gluoncv.model_zoo.get_model`
 2. Export the GluonCV model to get the symbol and parameter files using `export_block`