[commas] add section on additional trailing commas. fixes airbnb#75

Author: hshoff

README.md

@@ -17,7 +17,7 @@
   1. [Blocks](#blocks)
   1. [Comments](#comments)
   1. [Whitespace](#whitespace)
-  1. [Leading Commas](#leading-commas)
+  1. [Commas](#commas)
   1. [Semicolons](#semicolons)
   1. [Type Casting & Coercion](#type-coercion)
   1. [Naming Conventions](#naming-conventions)
@@ -802,9 +802,9 @@
 
     **[[⬆]](#TOC)**
 
-## <a name='leading-commas'>Leading Commas</a>
+## <a name='commas'>Commas</a>
 
-  - **Nope.**
+  - Leading commas: **Nope.**
 
     ```javascript
     // bad
@@ -834,6 +834,34 @@
     };
     ```
 
+  - Additional trailing comma: **Nope.** This can cause problems with IE6/7 and IE9 if it's in quirksmode. Also, in some implementations of ES3 would add length to an array if it had an additional trailing comma. This was clarified in ES5 ([source](http://es5.github.io/#D)):
+
+  > Edition 5 clarifies the fact that a trailing comma at the end of an ArrayInitialiser does not add to the length of the array. This is not a semantic change from Edition 3 but some implementations may have previously misinterpreted this.
+
+    ```javascript
+    // bad
+    var hero = {
+      firstName: 'Kevin',
+      lastName: 'Flynn',
+    };
+
+    var heroes = [
+      'Batman',
+      'Superman',
+    ];
+
+    // good
+    var hero = {
+      firstName: 'Kevin',
+      lastName: 'Flynn'
+    };
+
+    var heroes = [
+      'Batman',
+      'Superman'
+    ];
+    ```
+
     **[[⬆]](#TOC)**