Add usage with arguments example

Author: blakeembrey

README.md

@@ -22,6 +22,10 @@ flatten([1, [2, [3, [4, [5], 6], 7], 8], 9]);
 
 flatten([1, [2, [3, [4, [5], 6], 7], 8], 9], 2);
 //=> [1, 2, 3, [4, [5], 6], 7, 8, 9]
+
+(function () {
+  flatten(arguments); //=> [1, 2, 3]
+})(1, [2, 3]);
 ```
 
 ## License

package.json

@@ -16,7 +16,8 @@
   },
   "keywords": [
     "array",
-    "flatten"
+    "flatten",
+    "arguments"
   ],
   "author": {
     "name": "Blake Embrey",