## What changes were proposed in this pull request?

This PR tries to add a SparkR vignette, which works as a friendly guidance going through the functionality provided by SparkR.

## How was this patch tested?

Manual test.

Author: junyangq <qianjunyang@gmail.com>
Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>
Author: Junyang Qian <junyangq@databricks.com>

Closes #14980 from junyangq/SPARKR-vignette.