Update CHANGELOG.md

Author: vine77

packages/react-components/CHANGELOG.md

@@ -1,5 +1,6 @@
 # [Unreleased](https://github.com/puppetlabs/design-system/compare/@puppet/[email protected])
 
+- [sass-variables] Add breakpoint variables and media query mixins for use in responsive layouts (by [@caseywilliams](https://github.com/caseywilliams) in [#218](https://github.com/puppetlabs/design-system/pull/218))
 - [RadioButton] Add new Radiobutton component (by [@jilliankeenan](https://github.com/jilliankeenan) in [#215](https://github.com/puppetlabs/design-system/pull/215))
 - [Checkbox] Fix color of checked icon for error state (by [@jilliankeenan](https://github.com/jilliankeenan) in [#215](https://github.com/puppetlabs/design-system/pull/215))