initial commit of a migration-to-firebase doc

Author: tbozhikov

MIGRATE-TO-FIREBASE.md

@@ -0,0 +1,57 @@
+This is a migration guide which can help you switch your app from using push-plugin to using nativescript-plugin-firebase
+1. Go to your app's root folder and execute
+```bash
+tns plugin add nativescript-plugin-firebase
+```
+> Upon plugin installation, you'll be prompted to choose which features to use. Choose "yes" for Firebase Messaging (of course :)). By default, the plugin saves the configuration as a file (firebase.nativescript.json) to use it when reinstalling the plugin.
+
+2. Add `GoogleService-Info.plist` (for iOS) or `google-services.json` (for Android) in App_Resources/iOS (and App_Resources/Android, respectively). These are the configuration files that come from your Firebase apps. If you don't have such yet, go to https://console.firebase.google.com and create one. See [firebase plugin's docs]("https://github.com/EddyVerbruggen/nativescript-plugin-firebase/blob/master/docs/MESSAGING.md") for more info on initial setup.
+
+3. Add some code [to handle a notification]("https://github.com/EddyVerbruggen/nativescript-plugin-firebase/blob/master/docs/MESSAGING.md#handling-a-notification") 
+
+EXAMPLE: The following code using push-plugin:
+```js
+private pushSettings = {
+    notificationCallbackIOS: (message: any) => {
+        this.updateMessage("Message received!\n" + JSON.stringify(message));
+    }
+};
+
+pushPlugin.register(this.pushSettings, (token: String) => {
+    console.log("Device registered. Access token: " + token);
+
+    pushPlugin.registerUserNotificationSettings(() => {
+        console.log("Successfully registered for push.");
+    }, (err) => {
+        console.log(("Error registering for interactive push: " + JSON.stringify(err));
+    });
+}, (errorMessage: String) => {
+    console.log((JSON.stringify(errorMessage));
+});
+```
+... could be rewriten using firebase like:
+```js
+firebase.init({
+    onMessageReceivedCallback: (message: firebase.Message) => {
+        console.log(`Message: ${message}`);
+    },
+    onPushTokenReceivedCallback: function(token) {
+        console.log("Firebase push token: " + token);
+    }
+});
+```
+To test with sending messages, you can use the UI in Firebase Console, or use the  `https://fcm.googleapis.com/fcm/send` API. See the [testing docs section in firebase plugin]("https://github.com/EddyVerbruggen/nativescript-plugin-firebase/blob/master/docs/MESSAGING.md#testing").
+
+4. 
+
+-----------
+
+2.  You can go to demos and see there but below are how some usages of push-plugin translate to firebase:
+ 
+pushPlugin.areNotificationsEnabled(<some-callback-func>) -> firebase.areNotificationsEnabled(): boolean
+| push-plugin | nativescript-plugin-firebase |
+| :-------------: | :-------------: | 
+| pushPlugin.areNotificationsEnabled(<some-callback-func>)      | firebase.areNotificationsEnabled(): boolean |
+| 
+pushPlugin.register(this.pushSettings)
+pushPlugin.registerUserNotificationSettings()      | firebase.registerForInteractivePush(model) |