Adds documentation in the README about how to provide FB credentials

[esilvert]

Hello everyone,

Using FCM I've come across the same issue as #110.
We are a bit confused about how to pass the firebase credentials safely from an ENV variable.

Digging around the lib, I've found two things:

1. The limitation of passing a file path to FCM comes from googleauth gem that requires a json_key_io responding to #read (source).
2. This lib is clever enough to use any IO responding to #read as credentials instead of a file path

Only thing is that the usage of an IO is a bit hidden in the test suite here or in the codebase here.

So here I am suggesting adding a small part about it in the readme.

Feel free to ask for a rewording or more details.