## 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.
