[Docs] Clarify the need for "arrayLimit" option

README.md

@@ -182,7 +182,7 @@ assert.deepEqual(withIndexedEmptyString, { a: ['b', '', 'c'] });
 ```
 
 **qs** will also limit specifying indices in an array to a maximum index of `20`. Any array members with an index of greater than `20` will
-instead be converted to an object with the index as the key:
+instead be converted to an object with the index as the key. This is needed to handle cases when someone sent, for example, `a[999999999]` and it will take significant time to iterate over this huge array.
 
 ```javascript
 var withMaxIndex = qs.parse('a[100]=b');