[SPARK-18875][SPARKR][DOCS] Fix R API doc generation by adding `DESCRIPTION` file

## What changes were proposed in this pull request?

Since Apache Spark 1.4.0, R API document page has a broken link on `DESCRIPTION file` because Jekyll plugin script doesn't copy the file. This PR aims to fix that.

- Official Latest Website: http://spark.apache.org/docs/latest/api/R/index.html
- Apache Spark 2.1.0-rc2: http://people.apache.org/~pwendell/spark-releases/spark-2.1.0-rc2-docs/api/R/index.html



## How was this patch tested?

Manual.

```bash
cd docs
SKIP_SCALADOC=1 jekyll build
```

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #16292 from dongjoon-hyun/SPARK-18875.

(cherry picked from commit ec0eae48)
Signed-off-by: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

docs/_plugins/copy_api_dirs.rb

@@ -142,4 +142,7 @@ if not (ENV['SKIP_API'] == '1')
   puts "cp -r R/pkg/html/. docs/api/R"
   cp_r("R/pkg/html/.", "docs/api/R")
 
+  puts "cp R/pkg/DESCRIPTION docs/api"
+  cp("R/pkg/DESCRIPTION", "docs/api")
+
 end