Document how to configure push adapter with FCM

[mman]

New Issue Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.
• I can reproduce the issue with the latest versions of Parse Server and the Parse Server Push Adapter.

Issue Description

I had to look through code to figure out how to configure FCM.

Addressed by: #240

[parse-github-assistant]

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