Update User Documentation https://github.com/NervanaSystems/ngraph-mxnet/blob/master/NGRAPH_README.md

[Lam Thuy Nguyen]

Need to update the documentation due to new changes in design.

1. 3rdparty/ngraph is no longer valid.

2. Need to update the test report.

[Lam Thuy Nguyen]

Complete.