Merge pull request #29 from button/liwei/update-purchase-date

Add purchase_date to Readme

Author: LiweiButton

README.rst

@@ -182,6 +182,7 @@ Create
         'total': 50,
         'currency': 'USD',
         'order_id': '2007',
+        'purchase_date': '2017-07-25T08:23:52Z',
         'finalization_date': '2017-08-02T19:26:08Z',
         'btn_ref': 'srctok-XXX',
         'customer': {