message: Add instruction and example in README.md
diff: @@ -78,6 +78,22 @@ $ # In docker
  $ R -e "devtools::test()"
  ```
  +Alternatively, we also provided a reference Dockerfile [here](R-package/scripts/Dockerfile) for you
  +so that you can use the R package directly for testing. You can build it via:
  +```
  +docker build -t tfio-r-dev -f R-package/scripts/Dockerfile .
  +```
  +
  +Inside the container, you can start your R session, instantiate a `SequenceFileDataset`
  +from an example [Hadoop SequenceFile](https://wiki.apache.org/hadoop/SequenceFile)
  +[string.seq](R-package/tests/testthat/testdata/string.seq), and then use any [transformation functions](https://tensorflow.rstudio.com/tools/tfdatasets/articles/introduction.html#transformations) provided by [tfdatasets package](https://tensorflow.rstudio.com/tools/tfdatasets/) on the dataset like the following:
  +
  +```{R}
  +library(tfio)
  +dataset <- sequence_file_dataset("R-package/tests/testthat/testdata/string.seq") %>%
  + dataset_repeat(2)
  +```
  +
  ## License
  [Apache License 2.0](LICENSE)
  