Add the RCTNetwork to the podfile

Summary:
The pod `RCTNetwork` is missing from the `Podfile` documentation. Without it the view controller does not run.

This probably also closes #8449.
Closes https://github.com/facebook/react-native/pull/10389

Differential Revision: D4021432

Pulled By: JoelMarcey

fbshipit-source-id: 689ed540c6750bf4ccaee1e7943d9175ac50d6c5

docs/IntegrationWithExistingApps.md

@@ -209,6 +209,7 @@ target 'NumberTileGame' do
   pod 'React', :path => '../node_modules/react-native', :subspecs => [
     'Core',
     'RCTText',
+    'RCTNetwork',
     'RCTWebSocket', # needed for debugging
     # Add any other subspecs you want to use in your project
   ]
@@ -233,6 +234,7 @@ target 'swift-2048' do
   pod 'React', :path => '../node_modules/react-native', :subspecs => [
     'Core',
     'RCTText',
+    'RCTNetwork',
     'RCTWebSocket', # needed for debugging
     # Add any other subspecs you want to use in your project
   ]