docs(readme): update @smooth-ui/core-em installation instructions (#155)

Author: diegohaz

packages/core-em/README.md

@@ -11,7 +11,7 @@
 [![Small size][size-badge]][build-min]
 
 ```sh
-npm install @smooth-ui/core-em @emotion/core @emotion/styled emotion-theming
+npm install @smooth-ui/core-em reakit @emotion/core @emotion/styled emotion-theming
 ```
 
 Smooth UI is a style system / UI library for [React](https://reactjs.org/). It works with [Styled Components 💅](https://www.styled-components.com) or [Emotion 👩‍🎤](https://emotion.sh/).