Add key attribute in React repeat example

[just-boris]

While other frameworks can take care about repeating items, React requires special key attribute. There is a section in React documentation about this.

Unless you do it, you will get a warning from React in developer mode.

[jeffcarp]

Thank you @just-boris! I've heard that using index makes the diffing algorithm less efficient, do you think it would make sense to make it items.map(item => <div key={index.id}></div>)?

[just-boris]

@jeffcarp Updated PR.

This is okay for me. My goal is showing that key always necessary in React, rather than other frameworks

[jeffcarp]

Excellent, thanks for the contribution @just-boris 👍 🎉