add description to array includes snippet since it looks a lot like es6 native includes function.

snippets/array-includes.md

@@ -3,6 +3,7 @@
 Use `slice()` to offset the array/string and `indexOf()` to check if the value is included.
 Omit the last argument, `fromIndex`, to check the whole array/string.
 
+In [ES6](http://www.ecma-international.org/ecma-262/7.0/#sec-array.prototype.includes), you can use a similar native `.includes()` function.
 ```js
 const includes = (collection, val, fromIndex=0) => collection.slice(fromIndex).indexOf(val) != -1;
 // includes("30-seconds-of-code", "code") -> true