Update podspec integration instructions

Summary:
You need to add CxxBridge or BatchedBridge as a dependency while we have both. When we stop shipping the BatchedBridge in OSS, this requirement will disappear again.
Closes https://github.com/facebook/react-native/pull/15084

Differential Revision: D5446885

Pulled By: javache

fbshipit-source-id: aa0023cc08e97fc59e2071a3750f98026e34fd0d

docs/IntegrationWithExistingApps.md

@@ -244,6 +244,7 @@ target 'swift-2048' do
   # but if not, adjust the `:path` accordingly
   pod 'React', :path => '../node_modules/react-native', :subspecs => [
     'Core',
+    'CxxBridge', # Include this for RN >= 0.47
     'DevSupport', # Include this to enable In-App Devmenu if RN >= 0.43
     'RCTText',
     'RCTNetwork',