[MINOR][R][DOC] Fix R documentation generation instruction.

## What changes were proposed in this pull request?
changes in R/README.md

- Make step of generating SparkR document more clear.
- link R/DOCUMENTATION.md from R/README.md
- turn on some code syntax highlight in R/README.md

## How was this patch tested?
local test

Author: Kai Jiang <jiangkai@gmail.com>

Closes #13488 from vectorijk/R-Readme.