Update FCM steps in README.md

[nebbii]

Had some struggles setting up FCM in my repository with the given documentation, so I'm suggesting some extra steps people might look over on their initial tests. From the documentation it's not immediately clear that data is not where the title and body of a notification go. Leaving notification blank causes rpush to return a success, but nothing will actually arrive on the device. The project ID also might have changed labels overtime, as setting the "project ID" as my firebase_project_id instead of the "project number" also prevents notifications from showing up.