[Documentation] Details/Example for Indexes as keys

[ajcumine]

This aims to add more information to the documentation and address #79

1. Added more details on the performance and state issues cause by using indexes as keys in lists.
2. Added information that React uses indexes as keys if none are given.
3. Added a link to an example on Codepen to show the issue cause by using indexes as keys in list on state. http://codepen.io/ajcumine/pen/KmVWmQ?editors=0010
4. Added a link to an example on Codepen to show how not using indexes as keys in a list fixes the previous example's issues with state and reordering. https://codepen.io/ajcumine/pen/ZKQeJM?editors=0010

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[bvaughn]

The /react prefix should not be added to this link.

[bvaughn]

I actually preferred the original wording for this paragraph.

[bvaughn]

cc @gaearon to import these changes into his CodePen profile.

[ajcumine]

Thanks for the swift review @bvaughn, I've made the suggested change and fixed the link I had wrong.

[bvaughn]

I think these changes look reasonable. I prefer not to merge until we update the Codepen links to point at our standard account (codepen.io/gaearon) so I'll hand this issue to @gaearon for now. 😄

[bvaughn]

Hi @ajcumine. Sorry for dropping the ball on this PR. I couldn't merge it until we had an answer for the Codepen issue I mentioned previously. Now we do. As of PR #245, we're now able to directly link to code examples stored right here in this project's git repository.

If you'd be willing to update this PR to move the forked examples that you made on Codepen within this project (in the examples directory) then I'll be happy to review and merge it.

[ajcumine]

Hi @bvaughn. No worries, I'll update it in the next few days and get everything updated for you to review.

[ajcumine]

@bvaughn I found some time to get this done today :) It's all ready for your review, thanks again

[bvaughn]

Thanks a bunch!