Update bulk coupon docs (#1396)

* Add update functionality to the bulk coupon script

* Update create coupon script docs.

Author: malemburg

p3/management/commands/create_bulk_coupons.py

@@ -1,9 +1,9 @@
 
-""" Create a batch of single use discount coupons from a CSV file.
+""" Create/update a batch of discount coupons from a CSV file.
 
     Parameters: <conference> <csv-file>
 
-    Creates coupons based on the CSV file contents:
+    Creates/updates coupons based on the CSV file contents:
     
 	code		- coupon code
         max_usage 	- max. number of uses
@@ -14,6 +14,9 @@
 
     Use --dry-run to test drive the script.
 
+    Existing coupon codes will get updated by the script.  Indexing is by
+    code.
+
 """
 import sys
 import csv