RT DB Writing String

        //Getting instance
        FirebaseFirestore db = FirebaseFirestore.getInstance();

        //Creating Map for the values we want to store
        Map<String ,String> data = new HashMap<>();
        data.put("username","waqas");

        //Getting Reference to "users" collection
        //REMEMBER: at this point, no collection will be created
        CollectionReference collectionReference = db.collection("users");

        //Getting reference to "new doc" document
        //REMEMBER: at this point, no document will be created either
        DocumentReference documentReference = collectionReference.document("new doc");

        //Writing data
        //REMEMBER: here both collection and document will be created and fields and values we created in hash will be stored to that document.
        documentReference.set(data);