Updates README with --ignore-sass-mixins info

Author: zmoazeni

README.md

@@ -45,11 +45,14 @@ rulesets that have fewer matches.
 
     $ csscss -n 10 -v path/to/style.css # ignores rulesets with < 10 matches
 
-If you prefer writing in [sass](http://sass-lang.com/), you can also parse your sass/scss files.
+If you prefer writing in [Sass](http://sass-lang.com/), you can also parse your sass/scss files.
 
     $ gem install sass
     $ csscss path/to/style.scss
 
+Sass users may be interested in the `--ignore-sass-mixins`
+experimental flag that won't match duplicate declarations from including mixins.
+
 If you prefer writing in [LESS](http://lesscss.org/), you can also parse your LESS files.
 
     $ gem install less