summarize payment system

paymentSystem.md

@@ -1,4 +1,9 @@
-# payment system
+# Payment system
 
-## Append-only
-## Stability
+## Payment process
+1. Retrieve users that needs charging from SQS and corresponding info: subscription ID, charge fees. 
+2. Retrieve user payment method from MySQL.
+3. Prepare payment request based on obtained info.
+4. Check whether the request is pending (Such payment request is sent, but not finished due to server failure or third party payment problems), if yes, ignore (avoid paying twice).
+5. Send request to Third Party Payment, retry or failure process ( send payment failure back to SQS )
+6. If OK response from Third Party, update pending list, send confirmation back to SQS.