message: docs: add `IconButton` to migration doc
diff: @@ -352,6 +352,27 @@ with other icons:
  import { createIcon, MoonIcon, SunIcon } from "@chakra-ui/icons"
  ```
  +### Icon Button
  +
  +- We've unified the usage of all icon props to only accept a React element.
  + Update all icon names used in `icon` to the equivalent icon
  + React element.
  +
  +> Replacement logic: If `icon` is `email`, then replace it with
  +> `<EmailIcon/>` from Chakra.
  +
  +```diff
  +import { PhoneIcon } from "@chakra-ui/core"
  +
  +- <IconButton icon="phone">Call</IconButton>
  ++ <IconButton icon={<PhoneIcon />}>Call</IconButton>
  +```
  +
  +**This reduces the effort needed to use custom icons, eliminates TypeScript
  +errors, and reduces unused icons bloating your app.**
  +
  +- Renamed `variantColor` to `colorScheme`
  +
  ### Skeleton
  Renamed `colorStart` and `colorEnd` props to `startColor` and `endColor`
  