Update Readme information for MacOs

[siegfriedschaefer]

Your readme is missing a hint on how to get rid of the metro openssl issue if just cloning your repo as it is in main-branch. You have to switch to the "react-native-0-69" branch if you want to use node > 17 under MacOS.
Easy flow:
git clone https://github.com/BabylonJS/BabylonReactNativeSample.git webxr
cd webxr
git checkout react-native-0-69
npm install
npm run android
This hint saves newcomers like me a lot of time searching the web and trying not working solutions. Please update your readme accordingly.
Thank you!