Update Podfile documentation for RN >= 0.42.0

Summary:
I've updated the documentation that gives an example of Podfile adding a note that from React Native 0.42.0, users have to explicitly include Yoga in their Podfile.

I didn't want to modify existing `Podfile` from this documentation because they refer to `Podfile` from existing projects that do not use RN 0.42.0. So I just added a note and an example to explain how to include Yoga.

This PR is to fix the following issue: #12670
Closes #12728

Differential Revision: D4716172

Pulled By: hramos

fbshipit-source-id: 12a6e61b0d426c776a7cc2dea3ac22a50ffe56ef

Author: Maxime Aoustin

docs/IntegrationWithExistingApps.md

@@ -218,6 +218,8 @@ target 'NumberTileGame' do
     'RCTWebSocket', # needed for debugging
     # Add any other subspecs you want to use in your project
   ]
+  # Explicitly include Yoga if you are using RN >= 0.42.0
+  pod "Yoga", :path => "../node_modules/react-native/ReactCommon/yoga"
 
 end
 ```
@@ -243,6 +245,8 @@ target 'swift-2048' do
     'RCTWebSocket', # needed for debugging
     # Add any other subspecs you want to use in your project
   ]
+  # Explicitly include Yoga if you are using RN >= 0.42.0
+  pod "Yoga", :path => "../node_modules/react-native/ReactCommon/yoga"
 
 end
 ```