add exponentiation operator

doc/differences.md

@@ -78,6 +78,7 @@ better suited if you'd like a full ES6 environment with polyfills and all.
 | Constants                    | ✓     | ✓       | ✓              |        |        |             |
 | Default parameters           | ✓     | ✓       | ✓              | ✓      | ✓      |             |
 | Destructuring                | ✓     | ✓       | ✓              | ✓      | ✓      | ✓           |
+| Exponentiation operator      | ✓     | ✓       |                |        |        |             |
 | For-of                       | ✓     | ✓       | ✓              | ✓      | ✓      |             |
 | Generators                   | ✓     | ✓       |                | ✓      |        |             |
 | Generator comprehension      | ✓     | ✓       |                |        |        |             |

doc/features.md

@@ -130,6 +130,15 @@ var [a] = [];
 a === undefined;
 ```
 
+## Exponentiation operator ([experimental](usage.md#experimental)) ([spec](https://github.com/rwaldron/exponentiation-operator))
+
+```javascript
+var a = 2;
+a **= 2;
+
+var squared = 2 ** 2;
+```
+
 ## For-of
 
 ```javascript

doc/index.md

@@ -40,6 +40,7 @@ And it doesn't end here! To see all the ways you can use 6to5, check out the
  - [Constants](features.md#constants)
  - [Default parameters](features.md#default-parameters)
  - [Destructuring](features.md#destructuring)
+ - [Exponentiation operator](features.md#exponentiation-operator) ([experimental](usage.md#experimental))
  - [For-of](features.md#for-of)
  - [Generators](features.md#generators)
  - [Generator comprehension](features.md#generator-comprehension) ([experimental](usage.md#experimental))