text: Title: [DOCS] Specifying R version in runtime.txt does not seem to be mentioned
  Body: Hey 👋 I believe an R environment can now be described by the following in a `runtime.txt` file:
  
  ```
  r-3.5-YYYY-MM-DD
  ```
  
  But that doesn't seem to be reflected in the current documentation [here](https://repo2docker.readthedocs.io/en/latest/howto/languages.html#the-r-language)?