Adds basic table of contents for mixins.less. You know for the kids.

less/mixins.less

 //
 // Mixins
 // --------------------------------------------------
+//
+// Table of Contents
+//
+// UTILITIES
+// -------------------------
+// - Clearfix
+// - WebKit-style focus
+// - Center-align a block level element
+// - Sizing shortcuts
+// - Placeholder text
+// - Text overflow
+// - Requires inline-block or block for proper styling
+// - CSS image replacement
+//
+// CSS3 PROPERTIES
+// --------------------------------------------------
+// - Single side border-radius
+// - Drop shadows
+// - Transitions
+// - Transformations
+// - Animations
+// - Backface visibility
+// - Prevent browsers from flickering when using CSS 3D transforms.
+// - Box sizing
+// - User select
+// - For selecting text on the page
+// - Resize anything
+// - CSS3 Content Columns
+// - Optional hyphenation
+// - Opacity
+//
+// GRADIENTS
+// --------------------------------------------------
+// - Reset filters for IE
+// - Retina images
+// - Responsive image
+//
+// COMPONENT MIXINS
+// --------------------------------------------------
+// - Horizontal dividers
+// - Dividers (basically an hr) within dropdowns and nav lists
+// - Panels
+// - Alerts
+// - Tables
+// - List Groups
+// - Button variants
+// - Button sizes
+// - Pagination
+// - Labels
+// - Contextual backgrounds
+// - Typography
+// - Navbar vertical align
+// - Vertically center elements in the navbar.
+// - Progress bars
+//
+// RESPONSIVE UTILITIES
+// -------------------------
+// Grid System
+// -----------
+// - Centered container element
+// - Generate the extra small columns
+// - Generate the small columns
+// - Generate the medium columns
+// - Generate the large columns
+// - Framework grid generation
+// - Basic looping in LESS
+// - Form validation states
+// - Form control focus state
 
 
 // Utilities
@@ -1025,3 +1093,4 @@
     height: auto;
   }
 }
+