Adding note about calling sync on resume

Author: lostintangent

README.md

@@ -308,7 +308,17 @@ The simplest way to do this is to perform the following in your app's root compo
     codePush.sync();
     ```
 
-If an update is available, it will be silently downloaded, and installed the next time the app is restarted (either explicitly by the end user or by the OS), which ensures the least invasive experience for your end users. If an available update is mandatory, then it will be installed immediately, ensuring that the end user gets it as soon as possible. Additionally, if you would like to display a confirmation dialog (an "active install"), or customize the update experience in any way, refer to the `sync` method's [API reference](#codepushsync) for information on how to tweak this default behavior.
+If an update is available, it will be silently downloaded, and installed the next time the app is restarted (either explicitly by the end user or by the OS), which ensures the least invasive experience for your end users. If an available update is mandatory, then it will be installed immediately, ensuring that the end user gets it as soon as possible.
+
+If you would like your app to discover updates more quickly, you can also choose to call `sync` every time the app resumes from the background, by adding the following code (or something equivalent) as part of your app's startup behavior (e.g. your root component's `componentDidMount` method). You can call `sync` as frequently as you would like, so when and where you call it just depends on your personal preference.
+
+```javascript
+AppState.addEventListener("change", (newState) => {
+    newState === "active" && codePush.sync();
+});
+```
+   
+Additionally, if you would like to display an update confirmation dialog (an "active install"), cofigure when an available update is installed (e.g. force an immediate restart) or customize the update experience in any way, refer to the `sync` method's [API reference](#codepushsync) for information on how to tweak this default behavior.
 
 <a id="apple-note">*NOTE: While [Apple's developer agreement](https://developer.apple.com/programs/ios/information/iOS_Program_Information_4_3_15.pdf) fully allows performing over-the-air updates of JavaScript and assets (which is what enables CodePush!), it is against their policy for an app to display an update prompt. Because of this, we recommend that App Store-distributed apps don't enable the `updateDialog` option when calling `sync`, whereas Google Play and internally distributed apps (e.g. Enterprise, Fabric, HockeyApp) can choose to enable/customize it.*</a>