Update the README Examples section for RNTester

Summary:
The Examples directory no longer exists and the instructions in this part of the README were stale.

Follow the instructions to run RNTester on iOS and Android

(this actually fails for me currently on Android, but I believe the instructions are correct and the failure is related to something else)

Closes #14222
Closes https://github.com/facebook/react-native/pull/14229

Differential Revision: D5144514

Pulled By: shergin

fbshipit-source-id: 19ca173e8c5e91de7e1e53a21dbcaa0968da092b