Recommend switching to `kreait/firebase-php`

[jeromegamez]

Hey there 👋🏻!

I'm the maintainer of kreait/firebase-php.

Since this project hasn't received updates in ~3 years, I would like to suggest adding a note to the README, mark the composer package as abandoned and suggesting kreait/firebase-php as a replacement.

Being the maintainer of that project, I feel a little bit weird suggesting this, but I believe it would help people ensuring that their projects continue to work reliably, and to use other Firebase components, not just the Realtime Database.