Update README

README.md

@@ -30,3 +30,18 @@
 --product-price=12000
 
 ```
+
+### Register a payment method (cc for now)
+
+```
+(ucloud-cli) [test@ungleich ucloud-cli]$ UCLOUD_PAY_SERVER=http://127.0.0.1:5000 python ucloud.py user register-payment \
+ --name xxxxxxxxxxxx \
+ --realm xxxxxxxx \
+ --seed xxxxxxxxxx \
+ --cc --number 4242424242424242 \
+ --cvc 225 \
+ --expiry-month 08 \
+ --expiry-year 2022 \
+ --card-holder-name "The tester" \
+ --email "monty@python.com"
+```