Add section about mixins

In my previous commit, I revised this document to advise against using
@extend, which included removing the section about mixins because it
only existed to recommend against using argumentless mixins in favor of
@extend. However, this document feels a bit incomplete without some
guidance on mixin usage, so I am adding a few sentences here.

Author: lencioni

README.md

@@ -17,6 +17,7 @@
   1. [Sass](#sass)
     - [Syntax](#syntax)
     - [Ordering](#ordering-of-property-declarations)
+    - [Mixins](#mixins)
     - [Extend directive](#extend-directive)
     - [Nested selectors](#nested-selectors)
 
@@ -218,6 +219,10 @@ We recommend creating JavaScript-specific classes to bind to, prefixed with `.js
     }
     ```
 
+### Mixins
+
+Mixins should be used to DRY up your code, add clarity, or abstract complexity--in much the same way as well-named functions. Mixins that accept no arguments can be useful for this, but note that if you are not compressing your payload (e.g. gzip), this may contribute to unnecessary code duplication in the resulting styles.
+
 ### Extend directive
 
 `@extend` should be avoided because it has unintuitive and potentially dangerous behavior, especially when used with nested selectors. Even extending top-level placeholder selectors can cause problems if the order of selectors ends up changing later (e.g. if they are in other files and the order the files are loaded shifts). Gzipping should handle most of the savings you would have gained by using `@extend`, and you can DRY up your stylesheets nicely with mixins.