Add user-guide/project documentation

[mohamed-ali]

It's still not very clear how and when to use sparklingml. So, It would help to either add a doc/ section or at least, for now, add links to @holdenk video talks/sides on the project so that new users/contributors get a clearer picture on the project.

[mohamed-ali]

Probably this is one of the videos to link to:

• https://databricks.com/session/extending-apache-spark-ml-adding-your-own-algorithms-and-tools