mention KeyValue change in 2.2 changelog

[simonmichael]

Thanks for the new release.
https://hackage.haskell.org/package/aeson-2.2.0.0/docs/Data-Aeson-Types.html#t:KeyValue 's new e parameter should be mentioned in https://hackage.haskell.org/package/aeson-2.2.0.0/changelog I think ?

[georgefst]

Yep, this just caused me a lot of confusion (working on nbloomf/webdriver-w3c#64)!