update kargo bidder docs to clarify need for mobile device usage/emulation (prebid#841)

Author: samuelhorwitz

dev-docs/bidders/kargo.md

@@ -11,7 +11,7 @@ prebid_1_0_supported : true
 ---
 
 ### Note:
-Kargo is an invitation-only marketplace.  Please reach out to your Kargo account manager to get setup.
+Kargo is an invitation-only marketplace.  Please reach out to your Kargo account manager to get setup.  Also, you *must* test on a mobile device, or emulate a mobile device by manipulating the user agent string sent to the server.
 
 ### bid params