Update README.md

Author: blankg

README.md

@@ -29,6 +29,7 @@ A project committed to making file access and data transfer easier and more effi
  * [Multipart/form upload](#user-content-multipartform-data-example--post-form-data-with-file-and-data)
  * [Upload/Download progress](#user-content-uploaddownload-progress)
  * [Cancel HTTP request](#user-content-cancel-request)
+ * [iOS Background Uploading](#user-content-ios-background-uploading)
  * [Android Media Scanner, and Download Manager Support](#user-content-android-media-scanner-and-download-manager-support)
  * [Self-Signed SSL Server](#user-content-self-signed-ssl-server)
  * [Transfer Encoding](#user-content-transfer-encoding)
@@ -475,6 +476,34 @@ If you have existing code that uses `whatwg-fetch`(the official **fetch**), it's
 
 [See document and examples](https://github.com/joltup/rn-fetch-blob/wiki/Fetch-API#fetch-replacement)
 
+### iOS Background Uploading
+ Normally, iOS interrupts network connections when an app is moved to the background, and will throw an error 'Lost connection to background transfer service' when the app resumes. To continue the upload of large files even when the app is in the background, you will need to enable IOSUploadTask options.
+
+First add the following property to your AppDelegate.h:
+```
+@property (nonatomic, copy) void(^backgroundTransferCompletionHandler)();
+```
+Then add the following to your AppDelegate.m:
+```
+- (void)application:(UIApplication *)application
+handleEventsForBackgroundURLSession:(NSString *)identifier
+  completionHandler:(void (^)(void))completionHandler {
+  self.backgroundTransferCompletionHandler = completionHandler;
+}
+```
+The following example shows how to upload a file in the background:
+ ```js
+ RNFetchBlob
+    .config({
+        IOSBackgroundTask: true, // required for both upload
+        IOSUploadTask: true, // Use instead of IOSDownloadTask if uploading
+        uploadFilePath : 'file://' + filePath
+    })
+    .fetch('PUT', url, {
+            'Content-Type': mediaType
+        }, RNFetchBlob.wrap(filePath));
+```
+
 ### Android Media Scanner, and Download Manager Support
 
 If you want to make a file in `External Storage` becomes visible in Picture, Downloads, or other built-in apps, you will have to use `Media Scanner` or `Download Manager`.