[Docs] Document Platform in the modules section of the docs #3701

docs/PlatformSpecificInformation.md

@@ -0,0 +1,67 @@
+---
+id: platform-specific-code
+title: Platform Specific Code
+layout: docs
+category: Guides
+permalink: docs/platform-specific-code.html
+next: native-modules-ios
+---
+
+When building a cross platform app, the need to write different code for different platforms may arise. This can always be achieved by organizing the various components in different folders: 
+
+```sh
+/common/components/   
+/android/components/   
+/ios/components/
+```
+
+Another option may be naming the components differently depending on the platform they are going to be used in:
+
+```sh
+BigButtonIOS.js
+BigButtonAndroid.js
+```
+
+But React Native provides two alternatives to easily organize your code separating it by platform:
+
+## Platform specific extensions
+React Native will detect when a file has a `.ios.` or `.android.` extension and load the right file for each platform when requiring them from other components. 
+
+For example, you can have these files in your project:
+
+```sh
+BigButton.ios.js
+BigButton.android.js
+```
+
+With this setup, you can just require the files from a different component without paying attention to the platform in which the app will run.
+
+```javascript
+var BigButton = require('./components/BigButton');
+```
+
+React Native will import the correct component for the running platform.
+
+## Platform module
+A module is provided by React Native to detect what is the platform in which the app is running. This piece of functionality can be useful when only small parts of a component are platform specific.
+
+```javascript
+var {Platform} = React;
+
+var styles = StyleSheet.create({
+    height: (Platform.OS === 'ios') ? 200 : 100
+});
+```
+
+`Platform.OS` will be `ios` when running in iOS and `android` when running in an Android device or simulator.
+
+###Detecting android version
+On Android, the Platform module can be also used to detect which is the version of the Android Platform in which the app is running
+
+```javascript
+var {Platform} = React;
+
+if(Platform.Version === '5.0'){
+   console.log('Running on Lollipop!');
+}
+```

docs/Upgrading.md

@@ -4,7 +4,7 @@ title: Upgrading
 layout: docs
 category: Guides
 permalink: docs/upgrading.html
-next: native-modules-ios
+next: platform-specific-code
 ---
 
 Upgrading to new versions of React Native will give you access to more APIs, views, developer tools