Refine RNTester documentation to disable fabric

Summary:
Changelog: [Internal] Update the documentation to disable fabric in RN Tester.

RNTester build would fail if you previously built with fabric_enabled. This documentation adds the instruction to clean up before runing pod install.

In the future, we'd want to detect and do the clean up so that it doesn't fail in an obscure way.

Reviewed By: cortinico, dmitryrykun

Differential Revision: D33173173

fbshipit-source-id: a396eb71edc7bf174d0eb392799f45ae74274b76

Author: sota000

packages/rn-tester/README.md

@@ -18,6 +18,17 @@ If you are testing non-fabric component, modify [the fabric_enabled flag in RNTe
 fabric_enabled = false
 ```
 
+Also, if you previously built RNTester with fabric enabled, you might need to clean up the build files and Pods.
+```sh
+# Clean the generated files and folders to clean install RNTester
+cd packages/rn-tester/
+rm -rf build/generated/ios
+rm -rf Pods
+rm Podfile.lock
+```
+
+If you are still having a problem after doing the clean up (which can happen if you have built RNTester with older React Native versions where files were generated inside the react-native folder.), the best way might be to clean-install react-native (e.g. remove node_modules and yarn install).
+
 Both macOS and Xcode are required.
 - `cd packages/rn-tester`
 - Install [Bundler](https://bundler.io/): `gem install bundler`. We use bundler to install the right version of [CocoaPods](https://cocoapods.org/) locally.