[chore] add note about backendURLString

- Add test verve card

Author: ibrahimlawal

README.md

@@ -38,13 +38,24 @@ Before you can run the app, you need to provide it with your Paystack public key
 1. If you haven't already, sign up for a [Paystack account](https://dashboard.paystack.com/#/signup) (it takes seconds). Then go to https://dashboard.paystack.co/#/settings/developer.
 2. Replace the `paystackPublicKey` constant in ViewController.swift (for the Sample app) with your Test Public Key.
 3. Head to https://github.com/paystackhq/sample-charge-card-backend and click "Deploy to Heroku" (you may have to sign up for a Heroku account as part of this process). Provide your Paystack test secret key for the `PAYSTACK_TEST_SECRET_KEY` field under 'Env'. Click "Deploy for Free".
-4. Replace the `backendURLString` variable in the example iOS app with the app URL Heroku provides you with (e.g. "https://my-example-app.herokuapp.com")
+4. Replace the `backendURLString` variable in the example iOS app with the app URL Heroku provides you with (e.g. "https://my-example-app.herokuapp.com") **WITHOUT THE TRAILING '/'**
 
 ### Making a test Charge
 
-After completing the steps required above, you can make test payments through the app (use credit card number 4123 4501 3100 1381, along with 883 as cvc and any future expiration date) and then view the payments in your Paystack Dashboard!
+After completing the steps required above, you can (and should) test your implementation of the Paystack iOS library in your iOS app. You need the details of an actual debit/credit card to do this, so we provide ##_test cards_## for your use instead of using your own debit/credit cards. 
 
-And the return value from the backend will be displayed in your Output window.
+You will find test cards on [this Paystack documentation page](https://developers.paystack.co/docs/test-cards).
+
+To try out the OTP flow, we have provided a test "verve" card:
+
+```
+50666 66666 66666 6666
+CVV: 123
+PIN: 1234
+TOKEN: 123456
+```
+
+You can then view the payments in your Paystack Dashboard!
 
 ## Misc. notes