Add README.md description

Author: rogerhu

README.md

@@ -70,6 +70,21 @@ For more information about Parse and its features, see [the website][parseplatfo
 
   Assuming these major steps are done, adding the `parse-fcm-android` package will automatically instantiate a [ParseFirebaseJobService](https://github.com/parse-community/Parse-SDK-Android/blob/master/fcm/src/main/java/com/parse/fcm/ParseFirebaseJobService.java) that will register for a FCM token when the app starts.   See the setup instructions below to verify that FCM registration works.
 
+  One additional change you should make on the Parse server side [configuration](http://docs.parseplatform.org/parse-server/guide/#2-configure-parse-server) is to add an `fcm` key that matches your existing `android` senderId and apiKey.  This will ensure full compatibility of FCM should the Firebase [HTTP v1](https://firebase.google.com/docs/reference/fcm/rest/v1/projects.messages) in the future.
+
+  ```javascript
+  push: {
+    android: {
+      senderId: '', // The Sender ID
+      apiKey: '' // The Server API Key
+    },
+    fcm: {
+      senderId: '', // The Sender ID
+      apiKey: '' // The Server API Key
+    },
+  }
+  ```
+
 - **Option 2:** Compiling for yourself into AAR file
 
   If you want to manually compile the SDK, begin by cloning the repository locally or retrieving the source code for a particular [release][releases]. Open the project in Android Studio and run the following commands in the Terminal of Android Studio: