Documentation

[sxjscience]

After conducting more experiments on Distributed & Asynchronous training, we need to add some docs to help others use and understand our implementation. We can use http://www.sphinx-doc.org/en/stable/, which can be placed in https://readthedocs.org/ in the future.

[sxjscience]

@flyers @peterzcc @zmonoid For the doc-string, let's follow the Numpy style used in MXNet. http://sphinxcontrib-napoleon.readthedocs.io/en/latest/example_numpy.html. I'll add lint-check and revise the doc-strings gradually.