Consider adding deprecation/unmaintained warning to Readme

[jholloway7]

GCM is deprecated and being replaced by FCM on 2019-05-29.

• https://developers.google.com/cloud-messaging/android/android-migrate-fcm#update-server-endpoints
• https://firebase.google.com/docs/reference/admin/python/firebase_admin.messaging

There's also the upcoming Python 2.7 EOL in 2020.

It seems there likely won't be any plans to uplift this SDK for either situation. Just wondering if you'd be willing to commit a note in the README or whatever the Github process is for marking a project as deprecated/unmaintained.