Added link explaining danger of using index as key

content/docs/lists-and-keys.md

@@ -130,7 +130,7 @@ const todoItems = todos.map((todo, index) =>
 );
 ```
 
-We don't recommend using indexes for keys if the order of items may change. This can negatively impact performance and may cause issues with component state. If you choose not to assign a key to your list items then React will use indexes as keys. You may read an [in-depth explanation about why keys are necessary](/docs/reconciliation.html#recursing-on-children) if you're interested in more information.
+We don't recommend using indexes for keys if the order of items may change. This can negatively impact performance and may cause issues with component state. You may read Robin Pokorny's article for an [in-depth explanation on the negative impacts of using an index as a key](https://medium.com/@robinpokorny/index-as-a-key-is-an-anti-pattern-e0349aece318). If you choose not to assign a key to your list items then React will use indexes as keys. You may read an [in-depth explanation about why keys are necessary](/docs/reconciliation.html#recursing-on-children) if you're interested in more information.
 
 ### Extracting Components with Keys