Docs: Use another Sass compiler

docs/getting-started/compiling.md

@@ -13,11 +13,15 @@ From the command line:
 
 1. Install `grunt-cli` globally with `npm install -g grunt-cli`.
 2. Navigate to the root `/bootstrap` directory, then run `npm install`. npm will look at [package.json](https://github.com/twbs/bootstrap/blob/master/package.json) and automatically install the necessary local dependencies listed there.
+3. [Install Ruby][install-ruby], install [Bundler][gembundler] with `gem install bundler`, and finally run `bundle`. This will install all Ruby dependencies, such as Jekyll and Sass linter.
 
 When completed, you'll be able to run the various Grunt commands provided from the command line.
 
 **Unfamiliar with npm? Don't have node installed?** That's a-okay. npm stands for [node packaged modules](http://npmjs.org/) and is a way to manage development dependencies through node.js. [Download and install node.js](http://nodejs.org/download/) before proceeding.
 
+[install-ruby]: https://www.ruby-lang.org/en/documentation/installation/
+[gembundler]: http://bundler.io/
+
 ### Available Grunt commands
 
 #### Build - `grunt`
@@ -35,6 +39,20 @@ Builds and tests CSS, JavaScript, and other assets which are used when running t
 #### Watch - `grunt watch`
 This is a convenience method for watching just Sass files and automatically building them whenever you save.
 
+### Use another Sass compiler
+Bootstrap is compiled with [libsass][libsass] by default.
+Use another compiler by setting `TWBS_SASS` environment variable to:
+
+* `sass` to use [Ruby Sass][ruby-sass] via [grunt-contrib-sass][grunt-contrib-sass].
+* `libsass` (default) to use [libsass][libsass] via [grunt-sass][grunt-sass].
+
+For example, run `TWBS_SASS=sass grunt` to test and build Bootstrap with Ruby Sass.
+
+[ruby-sass]: https://github.com/sass/sass
+[grunt-contrib-sass]: https://github.com/gruntjs/grunt-contrib-sass
+[libsass]: https://github.com/sass/libsass
+[grunt-sass]: https://github.com/sindresorhus/grunt-sass
+
 ### Troubleshooting dependencies
 
 Should you encounter problems with installing dependencies or running Grunt commands, uninstall all previous dependency versions (global and local). Then, rerun `npm install`.