[SPARK-17317][SPARKR] Add SparkR vignette

## 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.

R/create-docs.sh

@@ -17,11 +17,13 @@
 # limitations under the License.
 #
 
-# Script to create API docs for SparkR
-# This requires `devtools` and `knitr` to be installed on the machine.
+# Script to create API docs and vignettes for SparkR
+# This requires `devtools`, `knitr` and `rmarkdown` to be installed on the machine.
 
 # After running this script the html docs can be found in 
 # $SPARK_HOME/R/pkg/html
+# The vignettes can be found in
+# $SPARK_HOME/R/pkg/vignettes/sparkr_vignettes.html
 
 set -o pipefail
 set -e
@@ -43,4 +45,9 @@ Rscript -e 'libDir <- "../../lib"; library(SparkR, lib.loc=libDir); library(knit
 
 popd
 
+# render creates SparkR vignettes
+Rscript -e 'library(rmarkdown); paths <- .libPaths(); .libPaths(c("lib", paths)); Sys.setenv(SPARK_HOME=tools::file_path_as_absolute("..")); render("pkg/vignettes/sparkr-vignettes.Rmd"); .libPaths(paths)'
+
+find pkg/vignettes/. -not -name '.' -not -name '*.Rmd' -not -name '*.md' -not -name '*.pdf' -not -name '*.html' -delete
+
 popd