Mention sass support in `css` argument of R Markdown format

[cderv]

rmarkdown 2.8 is out on CRAN now. I think it would be interesting to mention the support of .scss or .sass file in the css argument in this part of the doc: https://rstudio.github.io/sass/articles/sass.html#rmarkdown

It seems easier than using

```{r, echo = FALSE}
sass(sass_file("my-style.scss"))
```