add Sass documentation link to readme
# grass
-This crate aims to provide a high level interface for compiling Sass into
+This crate aims to provide a high level interface for compiling [Sass](https://sass-lang.com/documentation/) into
plain CSS. It offers a very limited API, currently exposing only 2 functions.
In addition to a library, this crate also includes a binary that is intended to act as an invisible
