Update README with new informations about Braintree configuration

If you don't create braintree configuration before proceed to checkout
you'll get an exception

README.md

@@ -73,7 +73,7 @@ SolidusPaypalBraintree::Gateway.new(
 ```
 
 ### Configure payment types
-Your payment method can accept payments in three ways: through Paypal, through ApplePay, or with credit card details entered directly by the customer. By default all are disabled for all your site's stores.
+Your payment method can accept payments in three ways: through Paypal, through ApplePay, or with credit card details entered directly by the customer. By default all are disabled for all your site's stores. Before proceeed to checkout, ensure you've created a Braintree configuration for your store:
 
 1. Visit /solidus_paypal_braintree/configurations/list