Firestore instructions need updating #235

Hi
I was trying to use the firestore option for this sample. I didn't find much instruction.

I unblocked myself by creating 3 collections: users, store and storeUsers

I also inserted the following rules to get me going. Clearly not secure. Any advice on how to further configure? I'll update this bug if I find a better solution.

rules_version = '2';
service cloud.firestore {
match /databases/{database}/documents {
match /{document=**} {
allow read, write: if true;
}
}}

Suggest we clarify further steps in the readme file and documentation for this.