Update PAYLOAD.md (#2602)

Creates PR in response to #2449

docs/PAYLOAD.md

@@ -2108,6 +2108,33 @@ Here's a sample JSON payload to send a push notification to an Android or iOS de
 }
 ```
 
+On iOS, using the FCM app server protocol, if you are trying to send a silent push and foreground pushes are not being triggered, try adding the ("content_available" : true) field to your payload. Here's the above sample JSON payload with this field included:
+```json
+{
+	"to" : "bk3RNwTe3H0:CI2k_HHwgIpoDKCIZvvDMExUdFQ3P1...",
+	/* To send a silent push, omit the entire notification section and send only data */
+	"notification": {
+		"title": "Test title", /* The notification's title */
+		"body": "Test body.", /* The notification's body text */
+		"subtitle": "Test subtitle", /* iOS: The notification's subtitle */
+		"sound": "default", /* The sound to play when the device receives the notification */
+		"tag": "1", /* Android: Group notifications with the same tag */
+		"icon": "push_icon", /* Android: PNG icon from the res/drawable folder */
+		"color": "#AABBCC" /* Android: Icon's background color in #RRGGBB format */
+	},
+	/* Optional payload that will be available from data.additionalData */
+	"data": {
+		"custom_var_1": "custom value here", /* Retrieved on app as data.additionalData.custom_var_1 */
+		"custom_var_2:" "custom value here" /* Retrieved on app as data.additionalData.custom_var_2 */
+	},
+  /* Forces FCM silent push notifications to be triggered in the foreground of your iOS device. */
+  "content_available": true  
+}
+```
+*Doc modification came in response to @andreszs - Issue [#2449](https://github.com/phonegap/phonegap-plugin-push/issues/2449).
+
+** IMPORTANT: When using the content_available field, Android payload issues may occur. [Read here](../docs/PAYLOAD.md#user-content-use-of-content_available-true) Make sure you separate your Android/iOS server payloads to mitigate any problems that may arise. 
+
 More information on how to send push notifications using the FCM HTTP protocol and payload details can be found here:
 
 - [Send messages using the legacy app server protocols](https://firebase.google.com/docs/cloud-messaging/send-message#send_messages_using_the_legacy_app_server_protocols 'Send messages using the legacy app server protocols')