Added react-height to readme

[reintroducing]

react-height was left out of the list of peerDependencies to install manually. I noticed this while installing react-collapse (warning for npm@3) so figured I'd just do a quick PR to update this.

[codecov-io]

Current coverage is 35.10%

Merging #29 into master will not affect coverage as of 08d136b

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  Missed          61     61       

Review entire Coverage Diff as of 08d136b

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[nkbt]

Thank you!

[reintroducing]

Sure thing :)

[just-boris]

Hello,

For me, it seems to have react-height as peer dependency is not a good idea. Usually, developers only put in their package.json only that packages, that they are actually using there. Unlike react-height which is not direct dependency, but now I have to care about its updates.

@nkbt could you change your mind and put react-height back into dev-dependency?

[nkbt]

Yeah I was thinking about it too. React-height seems to be react-collapse specific and not some sort of widely used library =). Makes sense to keep it as dep instead of peer