Add README instructions for Firestore and Realtime Database

Added instructions for exporting Firestore and Realtime Database access.

README.md

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+# Firestore Database
+**Export or Download your access account config**
+
+If your firebase use firestore you need to get your access account from your settings of firestore
+
+---
+
+# Realtime Database
+**Just put a ".json" in the end of url example:**
+
+https://nosql-demo-bd597-default-rtdb.firebaseio.com/.json
+
+**or if you need specific collection**
+
+https://nosql-demo-bd597-default-rtdb.firebaseio.com/users.json