[firebase_messaging] Updated README with Android "heads up" guide#2487

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MichaelM97 wants to merge 1 commit intofirebase:masterfrom

MichaelM97:firebase_messaging_readme_android_heads_up_update

Commits on May 4, 2020

Updated firebase_messaging README.md to explain how to display Android notifications as heads up
MichaelM97
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