Describe how we publish datasets

[zurk]

We do not have any guide about datasets publishing and it is worth to create it. The point of this PR is to describe how do we publish datasets right now to help newcomers better understand how to do it.

Resources that should be taken into account:

1. Draft design document "Documenting Models, Datasets, and Algorithms"
   
2. Proposal guide https://github.com/src-d/guide/pull/163/files

The similar issue about models: #59

[vmarkovtsev]

This happened in "source{d} datasets, a blog series" post which is scheduled for tomorrow.