Add Documents of MXNet.cpp to docs

[tqchen]

This project is getting valuable as people want to use compiled language for the project. I am opening this issue to see if you are interested in bring the documents to https://github.com/dmlc/mxnet/tree/master/docs/packages/c%2B%2B

This can help users to quickly get starting with mxnet.cpp!

[zhangchen-qinyinghua]

I will try to add some to the document within a few days.

[tqchen]

sounds great!

[zhangchen-qinyinghua]

Still not done because of some other work...trying to make this happen a.s.a.p....