better docs for TransitionGroup about use of key prop on children

[shelldandy]

Fixes #466

You don't always have the children of TransitionGroup mapped sometimes you want to render one at a time (eg image carousel) so it took me a while to figure out they need to have a key prop even when just rendering a single one by going to the source and reading this:

https://github.com/reactjs/react-transition-group/blob/master/src/utils/ChildMapping.js#L3-L19

So now im sending this PR with those insights so its easier for people to know about this

I made a small working example with styled-components

https://github.com/shelldandy/styled-transitions-example/blob/master/pages/index.js#L115-L121

[silvenon]

I always considered it kind of a hack to use a single child in TransitionGroup and modifying its key because then it's not a group (like the name implies). Instead you're just tricking TransitionGroup into thinking that it's a different element. But I'm fine with this until we potentially come up with a better API, I'd just like to make this more explicit in the documentation by explaining this technique.

[silvenon]

Thanks! The tests failing is just Jest's incompatibility with Node v11.11, nothing to worry about. jestjs/jest#8069

[jquense]

🎉 This PR is included in version 2.6.1 🎉

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