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.json

@@ -12,15 +12,13 @@
       "icon": "./assets/icons/react-navigation.png",
       "hideExponentText": false
     },
-    "sdkVersion": "18.0.0",
+    "sdkVersion": "20.0.0",
     "entryPoint": "./node_modules/react-native-scripts/build/bin/crna-entry.js",
     "packagerOpts": {
-      "projectRoots": "",
       "assetExts": [
         "ttf",
         "mp4"
-      ],
-      "config": "./rn-cli.config.js"
+      ]
     },
     "ios": {
       "supportsTablet": true