Fix the NavigationPlayground example! (#2170)

* [Playground] Commit auto-changes to package.json, yarn.lock

* [Playground] Rerun create-react-native-app

Preserving rn-cli.js and transformer.js

Still not quite working.

* [Playground] Fix the Expo loading issue by removing projectRoots

Now it looks in the right place to find its resources, including 'entryPoint'.

* [Playground] Add postinstall script to clean up files Yarn copies in

Yarn copies them when installing locally, causing lots of @providesModule collisions.

* Update the Contributors guide to explain how to use the example app

And recommend Yarn by default

* [Playground] Remove now-obsolete rn-cli.config.js

* [Playground] Upgrade expo and other dependencies

* [Playground] Don't delete .git just in case

examples/NavigationPlayground/App.test.js

@@ -1,9 +1,8 @@
 import React from 'react';
-import 'react-native';
-import renderer from 'react-test-renderer';
-
 import App from './App';
 
+import renderer from 'react-test-renderer';
+
 it('renders without crashing', () => {
   const rendered = renderer.create(<App />).toJSON();
   expect(rendered).toBeTruthy();