Add documentation on using the prepare_sliced_sparse_data API

[kalyc]

Summary

We added prepare_sliced_sparse_data API for being able to slice the sparse training data as MXNet doesn't support reshape Adding documentation to help users use the API

Related Issues

https://github.com/awslabs/keras-apache-mxnet/pull/170

PR Overview

• [X] This PR requires new unit tests [y/n] (make sure tests are included)
• [X] This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• [X] This PR is backwards compatible [y/n]
• [X] This PR changes the current API [y/n]