Update no-array-index-key.md to address issue 1123

jsx-eslint#1123

Author: penx

Diff for: docs/rules/no-array-index-key.md

@@ -95,3 +95,7 @@ things.reduceRight((collection, thing) => (
   collection.concat(<Hello key={thing.id} />)
 ), []);
 ```
+
+## When Not To Use It
+
+If there is nothing unique about the items, for example [you are breaking an array down in to chunks](https://github.com/yannickcr/eslint-plugin-react/issues/1123), then you may want to disable this rule with an override.