Better document/explain the importance of keys

[bvaughn]

Related issues:

• Document what happens if keys are not used at all facebook/react#9260
• Component keys and array index facebook/react#8410 reported by @bondarewicz

cc @ajcumine who submitted a PR to the old docs repo

[ajcumine]

I'll start working on this later today

[bvaughn]

This issue is all yours! 😄

I've added an "in-progress" label so that others will know not to start work on the issue. If you change your mind about the issue, no worries! Just let me know so that I can remove the label and free it up for someone else to claim.

Cheers!

[raunofreiberg]

@bvaughn Should probably close this issue and adjust the labels, seeing as it was resolved in #111.

[bvaughn]

Thanks for closing.

I don't think any label changes are required. :)