docs: install react-test-renderer typings (#181)

Author: thymikee

README.md

@@ -69,6 +69,12 @@ yarn add --dev react-native-testing-library
 
 This library has a peerDependencies listing for `react-test-renderer` and, of course, `react`. Make sure to install them too!
 
+Note for [Flow](https://flow.org) users – you'll also need to install typings for `react-test-rednerer`:
+
+```sh
+flow-typed install react-test-renderer
+```
+
 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.
 
 ## API / Usage