README ONLY: Realtime Database no longer has "test mode"

[chonghorizons]

Fixed my bug report/issue #41.

People who clone this great repo need to manually change the RealtimeDatabase Rules. This is a gotcha for newbies to the nomenclature of firebase. CloudFirestore != RealtimeDatabase.

(console.firebase.google.com has changed and deprecated/stopped promoting realtimeDatabase)

Also, small edit to the README.MD on the SHA1 key based on Issue #40.

PS: (I might try to rewrite this to use cloud_firestore since I'll be doing that for my project today, anyway.)