[SPARK-8554] Add the SparkR document files to `.rat-excludes` for `./dev/check-license`

[[SPARK-8554] Add the SparkR document files to `.rat-excludes` for `./dev/check-license` - ASF JIRA](https://issues.apache.org/jira/browse/SPARK-8554)

Author: Yu ISHIKAWA <yuu.ishikawa@gmail.com>

Closes #6947 from yu-iskw/SPARK-8554 and squashes the following commits:

5ca240c [Yu ISHIKAWA] [SPARK-8554] Add the SparkR document files to `.rat-excludes` for `./dev/check-license`