create a command for react keys

[CaptainTux]

the topic seems to come up quite frequently, either explicitly or implicitly
I think it would be useful to have a !keys command for such situations.
for example today I saw someone using UUIDs as keys which got generated on every render, and it wasn't the first time
perhaps something like (I'm bad with coming up with educational messages, forgive me, there should probably be another sentence or 2)

Keys are important if you map over an array and the callback returns JSX
some more here
For further information see

• https://beta.reactjs.org/learn/rendering-lists#keeping-list-items-in-order-with-key
• https://kentcdodds.com/blog/understanding-reacts-key-prop

[louis-young]

I think this is a great suggestion. Thanks for suggesting it and raising an issue @CaptainTux.

I've tweaked the blurb slightly and raised #263.