[SPARK-17736][DOCUMENTATION][SPARKR] Update R README for rmarkdown,…

## What changes were proposed in this pull request?

To build R docs (which are built when R tests are run), users need to install pandoc and rmarkdown. This was done for Jenkins in ~~[SPARK-17420](https://issues.apache.org/jira/browse/SPARK-17420)~~

… pandoc]

Author: Jagadeesan <as2@us.ibm.com>

Closes #15309 from jagadeesanas2/SPARK-17736.

docs/README.md

@@ -19,8 +19,8 @@ installed. Also install the following libraries:
     $ sudo gem install jekyll jekyll-redirect-from pygments.rb
     $ sudo pip install Pygments
     # Following is needed only for generating API docs
-    $ sudo pip install sphinx
-    $ sudo Rscript -e 'install.packages(c("knitr", "devtools", "roxygen2", "testthat"), repos="http://cran.stat.ucla.edu/")'
+    $ sudo pip install sphinx pypandoc
+    $ sudo Rscript -e 'install.packages(c("knitr", "devtools", "roxygen2", "testthat", "rmarkdown"), repos="http://cran.stat.ucla.edu/")'
 ```
 (Note: If you are on a system with both Ruby 1.9 and Ruby 2.0 you may need to replace gem with gem2.0)