Merge pull request airbnb#1303 from airbnb/lencioni-patch-1

Add link to Mixins section in TOC

Author: lencioni

react/README.md

@@ -6,6 +6,7 @@
 
   1. [Basic Rules](#basic-rules)
   1. [Class vs `React.createClass` vs stateless](#class-vs-reactcreateclass-vs-stateless)
+  1. [Mixins](#mixins)
   1. [Naming](#naming)
   1. [Declaration](#declaration)
   1. [Alignment](#alignment)