add pretty-format to peerDeps in readme

Author: thymikee

README.md

@@ -49,7 +49,7 @@ Open a Terminal in your project's folder and run:
 yarn add --dev react-testing-library
 ```
 
-This library has a peerDependencies listing for `react-test-renderer` and, of course, `react`. Make sure to install them too!
+This library has a peerDependencies listing for `react-test-renderer`, `pretty-format` and, of course, `react`. Make sure to install them too!
 
 As you may have noticed, it's not tied to React Native at all – you can safely use it in your React components if you feel like not interacting directly with DOM.