## What changes were proposed in this pull request?

Since `sparkR` is not used for submitting R Scripts from Spark 2.0, a user faces the following error message if he follows the instruction on `R/README.md`. This PR updates `R/README.md`.
```bash
$ ./bin/sparkR examples/src/main/r/dataframe.R
Running R applications through 'sparkR' is not supported as of Spark 2.0.
Use ./bin/spark-submit <R file>
```

## How was this patch tested?

Manual.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #11842 from dongjoon-hyun/update_r_readme.