[SPARK-18264][SPARKR] build vignettes with package, update vignettes for CRAN release build and add info on release

## What changes were proposed in this pull request?

Changes to DESCRIPTION to build vignettes.
Changes the metadata for vignettes to generate the recommended format (which is about <10% of size before). Unfortunately it does not look as nice
(before - left, after - right)

![image](https://cloud.githubusercontent.com/assets/8969467/20040492/b75883e6-a40d-11e6-9534-25cdd5d59a8b.png)

![image](https://cloud.githubusercontent.com/assets/8969467/20040490/a40f4d42-a40d-11e6-8c91-af00ddcbdad9.png)

Also add information on how to run build/release to CRAN later.

## How was this patch tested?

manually, unit tests

shivaram

We need this for branch-2.1

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #15790 from felixcheung/rpkgvignettes.