added docs for roundel alias (prebid#2808)

dev-docs/bidders/roundel.md

@@ -0,0 +1,24 @@
+---
+layout: bidder
+title: Roundel
+description: Prebid Roundel Bidder Adapter
+biddercode: roundel
+aliasCode: ix
+pbjs: true
+pbs: true
+userIds: identityLink, netId, fabrickId, zeotapIdPlus
+pbs_app_supported: true
+schain_supported: true
+gdpr_supported: true
+usp_supported: true
+media_types: banner, video
+gvl_id: 10
+prebid_member: yes
+---
+
+## Overview
+Roundel is an aliased bidder for Index Exchange and must be configured alongside the [IdentityLink](/dev-docs/modules/userId.html#identitylink) user ID module. 
+
+## Bid Params
+Bidder params for roundel follow the same structure as the Index Exchange bidder. Please reference configuration examples [here](/dev-docs/bidders/ix).
+