Mention using bundler to install cocoapods in RNTester README (#28873)

Summary:
This makes sure the proper version of cocoapods gets used, this will avoid noise in diffs if someone ends up updating pods with a different version.

## Changelog

[Internal] [Changed] - Mention using bundler to install cocoapods in RNTester README
Pull Request resolved: https://github.com/facebook/react-native/pull/28873

Test Plan: N/A

Differential Revision: D21519862

Pulled By: shergin

fbshipit-source-id: 3dc555bc3aee6bee10127ba5b5862302a63346c4

RNTester/README.md

@@ -14,8 +14,8 @@ Before running the app, make sure you ran:
 
 Both macOS and Xcode are required.
 
-- Install CocoaPods. We installing CocoaPods using [Homebrew](http://brew.sh/): `brew install cocoapods`
-- Run `cd RNTester; pod install`
+- Install [Bundler](https://bundler.io/): `gem install bundler`. We use bundler to install the right version of [CocoaPods](https://cocoapods.org/) locally.
+- Install Bundler and CocoaPods dependencies: `bundle install && bundle exec pod install`
 - Open the generated `RNTesterPods.xcworkspace`. This is not checked in, as it is generated by CocoaPods. Do not open `RNTesterPods.xcodeproj` directly.
 
 ### Running on Android