[DOCS] Specifying R version in runtime.txt does not seem to be mentioned

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)?