Add CODE_OF_CONDUCT.md to link to code of conduct (#3454)
Add CODE_OF_CONDUCT.md to link to code of conduct (#3454)

Links to same code of conduct already linked from readme

Signed-off-by: Prasanth Pulavarthi <4247628b926a7248ea57e412cecdead7fcaeb26c@microsoft.com>
onnx/onnx,onnx/onnx,onnx/onnx,onnx/onnx